Never heard of Thyago Vieira? That's OK. Until the 2016 season, he was a failed hard-throwing pitching prospect. But with polished mechanics and a slider to complement his triple-digit fastball, he's become the Mariners' top relief prospect going into next season.

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PEORIA, Ariz. — The question elicited similar reactions and answers from his various teammates.

“It’s intimidating,” said catcher Tyler Marlette. “He’s a really big dude and he gives it his all and he’s coming at you with every pitch. It’s very violent. It’s very nerve-racking for hitters. I think that’s what makes him so effective.”

Ask a fellow pitcher in the Mariners organization.

“The fastball comes out of his hand a little differently than most guys,” said reliever Emilio Pagan. “And it’s a not a straight fastball. It’s sink at 102 mph. It’s pretty unfair. He’s got a lot of moving parts in his delivery. I wouldn’t want to be in the box against him that’s for sure cause he’s also got an aggressive attitude to him too. It’s a very commanding presence on the mound.”

How about Mariners’ director of player development Andy McKay?

“He throws really hard,” he said. “There can’t be many people in professional baseball with a better arm. He throws enough strikes. I don’t think anyone would say there’s a ton of command, but there are plenty of strikes and the slider is hard to hit and really hard not to swing at.”

Ask an opposing scout: “That’s a special arm with big league velocity and stuff.”

So the question that had all these people, and many others, providing such gushing and descriptive answers: “If someone had never seen him pitch, how would you describe Thyago Vieira throwing off the mound?”

His own answer: “I attack the strike zone and I attack the hitters. I’m very aggressive with my fastball and my slider is getting better.”

Never heard of Vieira? Don’t question your baseball knowledge or fandom. Until this past season, he was just another guy in the Mariners’ organization — a pitcher with some talent, armed with only a blazing fastball and minimal intuition on where it might go once it left his hand.

But after a transformative 2016 season that earned him a trip to the prestigious Arizona Fall League, where he flashed a fastball that hit 103 mph on multiple occasions, and a spot on the 40-man roster, Vieira could follow the path of fellow relievers Edwin Diaz and Dan Altavilla and make the accelerated midseason move to the Major Leagues. He is the best and hardest-throwing relief prospect in the organization.

“I’m so excited,” Vieira said. “It’s been an unbelievable year for me. “

Standing in the MLB clubhouse of the Mariners’ spring training complex in Peoria — a place he’ll be when pitchers and catchers report on Feb. 14 — Vieira thought about how much had changed in the previous eighth months of baseball.

“This was a big year for me,” he said. “I just tried to stay focused on my job and focus on being more consistent.”

It was a season that almost didn’t happen with the Mariners.

Coming into spring training, Vieira was 23 and had never pitched above the Low-A level in five seasons in the organization. Signed as non-drafted free agent out of Sao Paulo, Brazil in 2010, he had two non-descript seasons in the Venezuelan Summer League, a decent year with Short-season Everett, posting a 3.84 ERA as a starter and then two forgettable seasons with Class A Clinton as a reliever. He had trouble staying healthy and throwing strikes.

General manager Jerry Dipoto admitted that the organization considered just moving on from Vieira and adding him to the list of a handful players that were released before the season.

“In spring training, there was a question as to whether he was going to make a team or not based on his age and general performance history,” Dipoto said.

Fortunately for the organization, they decided to take one more chance on Vieira. It didn’t hurt that he still had a fastball that sat at 99 mph and could touch 102 mph.

“Ethan Katz took him on as a priority subject,” Dipoto said. “He said, ‘How can I take this guy with this incredible arm and do something with him?'”

Katz was a newcomer to the Mariners’ organization and slated to be the pitching coach for Class A Bakersfield. But he was familiar to Dipoto having worked as a pitching coach in the Angels minor league system from 2013-2014. He also served as pitching coach for the LaCrosse Loggers in the Northwoods League for two seasons with McKay as the manager.

“I’m a little biased,” McKay said. “There is probably not a bigger fan of Ethan Katz than me. Ethan is incredibly disciplined and detailed on daily work routines. It’s hard for players not to get better there. I almost say, they don’t have a choice in the matter because the day is going to be so structured and there are not a lot of options for what’s going to happen.”

Vieira became Katz’s special project.

“I saw a big arm with a lot of potential that just needed a little more time to figure it out,” Katz said. “There was some stuff that he was doing that needed to be fixed to help him succeed.”

Katz knew plenty of other coaches had failed in harnessing Vieira’s raw alent and wild mechanics. But he believed in his process.

“It’s getting him to understand what he was doing,” Katz said. “Every individual is different. It was somewhat of an overhaul that started in spring training and he stayed back in Arizona to continue to work on it. It was to give him the understanding: ‘Ok, this is why you are struggling with consistency. And this is how you are going to get better.’ And he bought in.”

With his career following a similar path of one of his wayward fastballs, Vieira understood he was running out of opportunities.

“He bought in from spring training on and I just embedded him into the idea — there are no days off whatsoever,” Katz said. “Every day we are getting our work in and every day he was there. He was ready to go and he worked very hard. There were a lot of mechanics that we tried to enforce to make sure he maintained his athletic ability and really getting him back with the right direction — not flying open and making sure he was working more north and south keeping him straight to home plate.”

But the goal for Katz wasn’t just to clean up Vieira’s mechanics to throw more strikes with his triple-digit fastball. No, he needed a viable breaking ball as a second pitch. Vieira technically had a slider in his repertoire. But it wasn’t very effective or thrown very often.

“We really had to harp on him that he needed a second pitch,” Katz said. “Even when you are blessed with a big arm, hitters can and will time everything. But if you give them that second pitch, they are going to be very overwhelmed. So we went to work every day on his mechanics and that slider to get it to where he can be consistent with it and comfortable with it.”

The progression of Vieira’s slider was a product of the adjustments to his mechanics.

“In spring training there was less than a 50-50 shot if the ball would break or even spin in the appropriate manner,” Dipoto said. “That’s all delivery repetition. That’s what happens when you have a pitching coach that puts in the time and making sure that your stride is on line and you are on time coming through your delivery. If you are doing everything right back there over the rubber, it’s going to result in more consistency with your ball action. It was never about hey, ‘Hey we need to get a little more spin off the fingers.’ Ethan spent a fair amount of time working on the back of Vieira’s delivery. And it showed.”

The duo put in daily work all spring. After holding Vieira back at extended spring training for a few extra weeks, he was sent to join Katz in Bakersfield. The process continued.

“He’s an unbelievable pitching coach,’ Vieira said. “He was the best pitching coach of my life. He worked with me every single day. Before every game that I wasn’t supposed to pitch in, we worked on my fastball command and my slider.”

The progress wasn’t immediate. The success was incremental.

“It’s funny in his first outing he threw his fastball a lot and the inning was fine and I told him basically: ‘This is not okay. You have to throw your secondary stuff or it’s not going to develop,'” he said.  “So I forced him to throw his slider the next inning. I was calling it from the dugout. There was a runner on third base and that made him really uncomfortable and he threw it to the backstop about 15 feet up it. After that point, he realized, ‘I absolutely have to get better with this.'”

The forced usage of the slider resulted in increased comfort with each outing.

“I felt like in the first half of the season I couldn’t throw my slider in certain counts like 3-2,” he said. “I put it in my mind that I just have to put it in the zone. I know the guy is going to swing because it’s so different from my fastball. The guys are always getting ready for my fastball and if I show something different, the guy is going to swing.”

Knowing that successes were important verification for the work and the changes, Katz was careful with the situations he brought Vieira in to pitch.

“He trusted the information and the process,” Katz said. “But I think when the game started it was all a little bit much at first. He needed more reps. He needed consistent reps and everything would fall into place. Our biggest thing when he came to me in High A was not to rush him into situations. We wanted to put him into situations where he could succeed and not just throw him into the fire.”

Even in the soft-landing situations, Vieira still had a few shaky moments where the control disappeared. But after seven straight appearances without allowing an earned run in mid-June, where he pitched nine total innings, striking out 12 and walking four, it was time remove the training wheels. They had to see Vieira’s polished mechanics and newfound slider in high leverage situations to see if the success was real and repeatable.

It came on June 28 at San Jose. Brought in for a save situation, trying to protect a 3-2 lead in the bottom of the ninth, Vieira dominated in a quick 1-2-3 inning.

“He got a big save for us — it was his first save,” Katz said. “It was boom, boom, boom — nine pitches, three outs, two strikeouts. From that day on, the mound presence and belief and confidence, it all just starting coming and it came fast. Not only did he believe in it, but I think he started to understand how good he can be.”

Over his next 20 appearances, Vieira posted a 1.35 ERA with seven saves. In 26 1/3 innings pitched, he struck out 36 batters and walked seven, holding hitters to a .179 batting average and a .414 on-base plus slugging percentage.

“When I was young, it was just throw the ball to home plate and hope,” he said. “There was no plan. Now everything has changed. My mentality has changed. Now I can read the hitters. Everything is different for me, I took this as an opportunity.”

And it’s been beneficial for Vieira and the Mariners.

“He’d never really had any success,” McKay said. “He’d never been able to sustain anything. I just think it was issues of getting with somebody that A. believed in him and B. created a process that he could follow and mandate that he did. It would be hard to be better than Vieira was for the majority of our season.”

Katz tried to downplay his role in it, saying Vieira “worked his butt off to make it happen.”

But Vieira was adamant in his praise.

“I have to say thank you to the Mariners for giving me the opportunity to work with him,” he said.

Dipoto credited both for the success. He now has a developing power relief prospect from a pitcher that was almost released.

“Ethan found a program that worked, really just a little TLC,” Dipoto said. “That’s what coaches do. And Vieira put in the work in every day. Every guy that gets to the big leagues and finds success is going to be able to pull the fish line back and get to a coach that made a difference in his life somewhere. My guess is when Vieira reaches that level, he is going to take a look back and say, ‘I’m really thankful for that time I spent with Ethan Katz.'”

Could that happen this season? Could Vieira be another power arm in the Mariners’ bullpen to go with Diaz and Altavilla? Dipoto believes there’s a chance.

“Coming to the big leagues fast is a dream,” Vieira said.

And then everyone will know what Thyago Vieira looks like throwing off the mound.