SANTA CLARA, Calif. — Jimmy Garoppolo's addition last season immediately turned around the fortunes of the San Francisco 49ers. Now the 49ers will have to go back to life before Garoppolo for the rest of the season.

The Niners announced Monday that Garoppolo tore the ACL in his left knee while making a cut late in a loss at Kansas City, sidelining him for the rest of the season.

The injury deals a serious blow to the 49ers (1-2), who had planned their rebuild around Garoppolo and now must turn again to C.J. Beathard at quarterback.

The Niners struggled with Beathard and Brian Hoyer last season before the midseason addition of Garoppolo changed their fortunes. San Francisco won the final five games last year after Garoppolo took over as starter and the team rewarded him with a $137.5 million, five-year contract.

The 49ers will have to wait another year to earn dividends on that investment. Garoppolo got hurt in the fourth quarter of a 38-27 loss Sunday when he was scrambling toward the sideline and decided to cut up field instead of going out of bounds.

Garoppolo's left knee buckled just before Kansas City cornerback Steven Nelson delivered a big hit, ending what was supposed to be his first full season as a starter after just three games.

Expectations were high in San Francisco this season that the team could end a four-year playoff drought after Garoppolo had a full off-season to immerse himself in coach Kyle Shanahan's offence. But with Garoppolo and new running back Jerick McKinnon both knocked out for the season with torn ACLs in September, those high hopes have been put on hold.

Garoppolo had completed 59.6 per cent of his passes with five TDs, three interceptions and an average of 8.1 yards per attempt for a 90 passer rating. While he hasn't been quite as efficient as he was late last year after being acquired at the trade deadline from New England for a second-round pick, the Niners offence has been far more potent than it was before he took over as quarterback.

Beathard started five games as a rookie last year, completing 54.9 per cent of his passes with four touchdowns, six interceptions, 19 sacks, 6.4 yards per attempt and a 69.2 passer rating that was second lowest in the NFL.

He threw a touchdown pass to George Kittle on his first snap Sunday only to have it negated by a penalty and wasn't in for any other plays in the game.

"It's huge having already played last year in six games," he said after the game. "It really helps having gone through it and having that experience. It's not my first time in the regular season. This time around it'll be a little easier."