SANTA CLARA, Calif. — For most of the season, Jimmy Garoppolo hasn’t been asked to carry too heavy a load at quarterback for the San Francisco 49ers thanks to a dominant defense and big-play running game.

The fact that Garoppolo was able to step up last week when those two facets weren’t performing as usual is a good sign for San Francisco heading into the much more difficult second half to the season.

The Niners (8-0) know they will need more games like that one at Arizona from their franchise quarterback if they are going to make a playoff run.

“I was very impressed,” Coach Kyle Shanahan said Thursday. “I thought he played his best game of the year. He made a lot of really good throws in rhythm and made a lot of off-schedule throws where he had to move in the pocket and generate some plays on his own. It was a very impressive game and it would have been very tough to win that game if he didn’t play that way.”

Garoppolo will try to build on that Monday night when San Francisco hosts Seattle in the most meaningful matchup between these rivals in five years.

Garoppolo made his 49ers debut against the Seahawks nearly two years ago, when he replaced an injured C.J. Beathard in the closing seconds of a 24-13 loss. Garoppolo completed his first two passes for San Francisco in that game, including a meaningless touchdown to Louis Murphy.

But he wasn’t able to play against Seattle last year after injuring his knee in Week 3 and will make his first start in the NFC West rivalry this week.

“It seems like a lifetime ago,” Garoppolo said of his Niners debut. “It’s a great group year in and year out. They’re always one of the top teams and it’ll be a great challenge for us.”

Garoppolo has the highest completion percentage (70.8 and passer rating (100.6) in his three seasons with the 49ers but he has been almost a complementary player at times this season on an offense running the ball a league-high 55.9 percent of the time.

But San Francisco needed a lot more from Garoppolo last week at Arizona when the defense was gassed in the second half and the running game stalled.

Garoppolo went 28 for 37 for 317 yards and no turnovers, while posting San Francisco’s second four-TD game in the past 16 seasons.

He was at his best on key downs, going 13 for 15 for 160 yards and all four of his TD passes on third or fourth down.

“Jimmy is awesome,” new teammate Emmanuel Sanders said. “He doesn’t have an ego. He comes in and works his butt off. He’s a high energy guy. He’s the type of person you enjoy being around.”

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