He was struggling with the LumberKings, hitting .157 (17 for 108) with six doubles, no homers and 13 RBI in 28 games. Jackson had a .240 on-base percentage with a .240 slugging percentage. He had also struck out 35 times with just six walks.

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TORONTO – For the second straight season, an injury has put Alex Jackson’s season on hold.

Jackson, the team’s top draft pick from the 2014 draft (No. 6 overall), was optioned back to extended spring training from Class A Clinton earlier this week.

He was struggling with the LumberKings, hitting .157 (17 for 108) with six doubles, no homers and 13 RBI in 28 games. Jackson had a .240 on-base percentage with a .240 slugging percentage. He had also struck out 35 times with just six walks.

On this date in 1995

Mike Blowers joined Alvin Davis as the second player in Mariners’ history to drive in eight runs in one game. Blowers went 4-for-5 with four extra-base hits in Seattle’s 15-6 over the Red Sox at the Kingdome. He drove in a run in his only at-bat that wasn’t hit, to go with a three-run double, a three-run triple and a two-run homer. Blowers entered the game hitting .118. The Mariners improved to 13-12 on the season and were in third place in the American League West, 3½ games behind the first-place Angels.

But it wasn’t just about poor numbers as the cause of the move. Jackson was bothered by an aching left shoulder, and Mariners director of player development Chris Gwynn decided it was time to shut him down.

“His left shoulder has been bothering him,” Gwynn said via phone from Arizona. “He hurt it in spring training diving for the ball.”

That injury came at the end of spring training in a minor-league game. Jackson had been scheduled to play in some Cactus League games with the Mariners near the end, but he was scratched in order to heal up for the season. But the pain in the shoulder started to return after the rigors of playing daily.

“We just want to make sure he’s healthy, so we decided to bring him back here,” Gwynn said.

Jackson will meet with the Mariners’ medical staff to have the shoulder checked out more. He will undergo treatment and strengthen the shoulder before returning to baseball activity.

“We’ll assess it when we deem him healthy,” Gwynn said.

Was the shoulder affecting Jackson’s swing and production at the plate?

“I don’t know, probably,” Gwynn said. “It’s a tough leap for a high-school kid going to this league, a lot of guys would struggle.”

Jackson, 19, played a partial season in the Arizona rookie league, but a broken jaw suffered from a line drive to the face sidelined him for much of it. The Mariners felt that his advanced approach and baseball maturity would allow him to skip Class A Everett and go directly to full-season Clinton. But he has struggled with the advanced competition. The injury also played into it. And the Mariners decided not to take any chances.

“The first thing is being totally healthy, because when you aren’t totally healthy one thing affects another thing and the next thing you know you have bad mechanics and bad habits,” Gwynn said. “We just want to stay away from that. So we are going to make sure he’s healthy before going back on the field.”

Gwynn could sense some frustration from Jackson with the injury and the struggles.

“He’s doing OK,” Gwynn said. “He’s never really struggled, ever. I’m sure that’s been different for him. He’s a 19-year-old kid. He loves to play and can’t wait to get back on the field.”

A look around the organization:

Class AAA Tacoma

During the course of any season, there’s always a player at Class AAA who has fans demanding he be called up to help the big club out. Earlier in the season it was shortstop Chris Taylor, for a while it was designated hitter Jesus Montero, but now shortstop Ketel Marte has become the latest object of desire. The 21-year-old switch-hitter leads the Pacific Coast League in hitting with a .359 (59 for 167) batting average this season. Marte has nine doubles, a triple, two homers and 22 RBI as well as 14 stolen bases.

With Taylor hitting under .200 since being called up, fans want Marte to get a chance. But is he ready?

Besides being very young, there are questions about his defense. Marte struggled with consistency this spring, committing several errors early and has seven total in 42 games this season. Opposing scouts believe he projects more as a second baseman. They have questions about whether his arm strength and slower release are conducive to shortstop.

Class AA Jackson

Young right-hander Edwin Diaz was promoted to the Generals after dominating in the Class A Cal League. Diaz, who was the Mariners’ minor league co-starting pitcher of the year last season, made seven starts for Bakersfield, going 2-0 with a 1.70 ERA. He allowed just seven runs in 37 innings in the hitter-friendly league.

Now he’s facing some of baseball’s top prospects in the Southern League.

“It was time,” Gwynn said. “It’s time for him to be tested.”

Diaz, 21, has a mid-90s fastball and solid offspeed pitches, but he will need more than just stuff to get outs at a higher level.

“I saw him pitch in Bakersfield and he was really good,” Gwynn said. “But I think it’s time for him to learn to put the ball where it’s supposed to be, not just rely on velocity. He’s worked on his change­up and his slider and he’s actually starting to pitch, not throw. Double A should be good for him. He’ll learn something every time he goes out. He needs better competition to keep evolving.”

The first two starts have been ugly: He has given up 10 runs in 92/3 innings pitched.

“He’ll adjust,” Gwynn said.

Class A Bakersfield

Tyler O’Neill can’t bench press the world. It just looks that way. The Canadian-born slugger is probably one of the strongest, most put-together players in the Mariners’ farm system. It’s not surprising his father was the winner of the Mr. Canada bodybuidling competition in 1975.

But O’Neill uses his muscles for more than posing. He can hit with power. The outfielder, 20, has belted 11 homers in 36 games for the Blaze this season.

“Oh man, I’ve seen him hit some of the farthest balls you’ll ever see,” Gwynn said.

But …

“We are trying to work with him on creating some plate discipline and get some better pitches to hit,” Gwynn said.

O’Neill has struck out 46 times and walked just seven times, while hitting .236.