SANTA CLARA, Calif. >> San Francisco 49ers cornerback Chris Culliver will miss the season with a torn ACL in his left knee.

Culliver was injured Thursday during a non-contact drill. Defensive coordinator Vic Fangio said Friday that the loss of the third-year veteran “puts stress” on a cornerback group that faltered late last season and in the playoffs during San Francisco’s run to the Super Bowl.

The 49ers brought in four-time All-Pro Nnamdi Asomugha in April to augment the position behind starters Carlos Rogers and Tarell Brown. The team returned its top five cornerbacks from last season.

Culliver was San Francisco’s third cornerback the past two seasons and played a key role in the team’s coverage packages, often taking over at left cornerback when the 49ers went to their nickel defense.

Now the 49ers have to groom somebody else for that role, which could affect how the team uses even its projected starters this season.

“Obviously we took a shot (Thursday) and it puts a stress on everybody else in that group,” Fangio said. “We’ve got the guys that were here last year and we brought in Nnamdi to try and help shore up that group, and right now we’re just going to have to see how it sorts itself out here in the preseason games and through the rest of practices.”

Culliver was San Francisco’s biggest cornerback last year and often was used in press coverage on the left side when the 49ers went with more than their regular four starting defensive backs. San Francisco, which ranked third in the NFL in total defense last year, went with extra defensive backs on nearly 70 percent of its defensive plays last season.

Fangio said regular right cornerback Brown could get a look on the left side and also covering slot receivers in nickel pages. Regular left cornerback Rogers usually covered slot receivers when the 49ers used three cornerbacks in coverage last season.

Holdover veterans Tramaine Brock and Perrish Cox also could move up in the pecking order at cornerback. Brock had two interceptions in 2011 while getting regular action as the team’s third cornerback before losing the job to Culliver after a hand injury.

“Now may be an opportunity for him to go out there and show what he’s got and take his shot,” Fangio said.

Cox had 14 tackles and knocked down two passes in his first year with the 49ers last season.

Despite finishing fourth in the NFL in pass defense last season, San Francisco’s pass coverage had some rocky performances near the end of the year. Green Bay’s Aaron Rodgers, Atlanta’s Matt Ryan and Baltimore’s Joe Flacco combined to torch the Niners for 940 yards and eight touchdown passes in three postseason games.

The 49ers then signed Asomugha, entering his 11th NFL season, to a one-year deal and also traded a conditional draft pick to Tampa Bay on July 19 to acquire veteran cornerback Eric Wright. That trade was voided when Wright failed his team physical.

At 6-foot-2 and 210 pounds, Asomugha has ideal size to handle Culliver’s role, and he built a reputation as a top cover corner during his first eight seasons with the Oakland Raiders. But that reputation took a hit the past two seasons after Asomugha signed a five-year, $60 million deal with the Philadelphia Eagles as one of the NFL’s top free agents of 2011.

The Eagles released Asomugha in March, and the 49ers are still determining exactly where he fits in now that Culliver is out.

“We really didn’t know what we were getting when we got Nnamdi,” Fangio said. “He’s got some good days out here and some days where we weren’t sure if he was going to be able to still have it. So I think we’re kind of in between with him right now. Hopefully, he’ll still have some gas left in his tank to go out there and play like he did prior to going to Philadelphia. So I think the jury is still out there.”

Fangio said the 49ers are unlikely to bring in another veteran free agent at this stage of the summer. San Francisco also drafted cornerback Marcus Cooper in the seventh round this spring, and first-round pick Eric Reid and free-agent veteran Craig Dahl are newcomers at safety who also could figure in coverage packages.

Culliver’s injury is one of several Fangio has had to deal with as he adjusts his defense this summer. All-Pro middle linebackers Patrick Willis (hand) and NaVorro Bowman (hamstring) are both on the mend from injuries and are missing practice, and Pro Bowl outside linebacker Aldon Smith is a recent addition to that list.

Rookie defensive linemen Tank Carradine (knee), the team’s second-round pick, and fifth-rounder Quinton Dial (toe) both are on San Francisco’s active/non-football injury list.

“Hopefully, it won’t press on into the season,” Fangio said. “The good part of it is some of these younger guys will get a lot more reps and be more prepared if they’re called on.”