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 offseason to immerse himself in coach Kyle Shanahan’s offense.