At 8-0, the San Francisco 49ers are off to their best start since 1990.

At 7-2, the Seattle Seahawks are off to their best start since 2013.

The old rivals will meet for the first time this season Monday night in Santa Clara, Calif., and their combined .882 winning percentage is the third-best ever for a “Monday Night Football” game in Week 10 or later, according to ESPN.

The 49ers opened as a 6.5-point favorite at home.

Five things to know about San Francisco:

1. The 49ers have the NFL’s No. 1 defense

They’re allowing 241.0 total yards per game and 138.1 yards per game passing, both league bests.

But the San Francisco defense will be without one of its top players, inside linebacker Kwon Alexander, who tore a pectoral muscle in a 28-25 victory at Arizona on Thursday. He’s expected to miss the remainder of the season.

Dre Greenlaw, a rookie fifth-round pick out of Arkansas, takes over for Alexander at weakside linebacker. Greenlaw has 19 tackles and one sack this season.

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2. They have the NFL’s No. 2 rushing offense

In just eight games, the 49ers have had more rushing attempts, 303, than anyone in the league, and their 37.9 runs per game are the most in the NFL since 2009 (the New York Jets also averaged 37.9 in 2009).

San Francisco’s 171.1 yards per game is second behind Baltimore’s 204.9 this season

Matt Breida (524 yards, 1 TD) and Tevin Coleman (355, 5 TD) lead the way for the 49ers’ rushing attack, with third-stringer Raheem Mostert (307 yards) leading the NFL with 5.6 yards per carry.

3. They are putting pressure on opposing QBs

Like, a lot of pressure.

The 49ers are pressuring quarterbacks on 31.8% of opponents’ dropbacks. That ranks No. 1 in the NFL.

The Seahawks, for comparison, are pressuring quarterbacks on 15.5% of opponents’ dropbacks. That ranks 31st of 32 NFL teams.

San Francisco has four players who rank among the NFL’s top 50 in sacks, led by rookie Nick Bosa’s seven sacks in his first eight games.

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Arik Armstead and Dee Ford each have 5.5 sacks, and DeForest Buckner has 4.0.

No one from Seattle’s defense ranks in the league’s top 50 and the Seahawks’ defensive linemen have combined for just nine sacks in nine games. Outside linebacker Mychal Kendricks leads Seattle with three sacks.

4. Richard Sherman is cranky

The 49ers allowed season highs in points, rushing yards (153) and total yards (357) Thursday at Arizona, and the Cardinals nearly came all the way back from a 14-point fourth-quarter deficit.

Which did not sit well with Richard Sherman.

“That was not championship football,” the ex-Seahawks cornerback said.

Sherman even owned up to a missed tackle of his own in the fourth quarter during an 88-yard touchdown catch-and-run by Arizona rookie Andy Isabella.

“We need to be humble,” Sherman said. “That was a humbling game. We need to be humble on all levels.”

5. Jimmy Garoppolo is coming off the best game of his career

Garoppolo went 28 for 37 for 317 yards with a career-high four touchdowns and a career-high passer rating of 136.9 against Arizona. He was even better on third and fourth downs, going 13 for 15 for 160 yards with all four of his TD passes.

Overall this season, Garoppolo is completing 70.8% of his passes for 1,806 yards, 13 TDs, 7 INTs and a rating of 100.6.

Garoppolo has established a quick connection with new receiver Emmanuel Sanders (11 receptions, 137 yards, two TDs in two games), acquired in a trade with Denver on Oct. 22. Former Husky receiver Dante Pettis has 11 catches for 109 yards and two TDs.

San Francisco’s offense is also getting healthier this week. Tight end George Kittle (the team’s leading receiver), fullback Kyle Juszczyk and starting tackles Joe Staley and Mike McGlinchey could all be back for Monday night.