49ers safety Eric Reid is out for Thursday’s game against the Los Angeles Rams and likely beyond with a posterior cruciate ligament injury in his left knee, coach Kyle Shanahan said Monday.

Reid underwent an MRI on Monday morning to reveal the extent of the injury, which Shanahan said will not require surgery.

“So it’ll heal on its own,” Shanahan said in a conference call Monday afternoon. “But he’s definitely not going to play this week and I’d be surprised … it’s probably going to be a little bit longer than that, also.”

Sunday against Seattle, Reid had a game-high six tackles before he sustained non-contact, left knee injury 4:39 before halftime.

Reid pulled up in coverage on a third-down incompletion toward the other side of the field. He returned for the start of the second half but twice went down upon aggravating his injury.

Asked why Reid kept re-entering the game when it seemed he wasn’t fully healthy, Shanahan said, “They kept looking at his knee. They didn’t feel that there was anything torn, so (Reid) felt it was alright.

“I haven’t gotten to talk to him personally yet, so I’m not exactly sure. But, when he checked out, it seemed alright. The X-ray they had looked good, and he said he could go so he kept trying to go and then he had his final straw and said he couldn’t go anymore. I guess that’s what happened.”

The 49ers’ depth at safety might be tested this week.

Jaquiski Tartt, who was on the field for all defensive 82 snaps against the Seahawks, has a neck ailment and is questionable to play against the Rams, Shanahan said.

The 49ers did not practice Monday. Had they practiced, Tartt would have been out. The other player in that group was tight end George Kittle (hip).

“I know he was hurting today,” Shanahan said of Tartt. “We’ll see how he is when he comes in (Tuesday).”

Tartt, assuming he can play, will likely be the 49ers’ strong safety and Jimmie Ward will play free safety. Ward, who missed the opener with a hamstring injury, was pressed into duty when Reid was out of Sunday’s game. Ward was originally slated to play only free safety in the dime package. Like our San Francisco 49ers Facebook page for more 49ers news, commentary and conversation.

With Ward, Reid and Tartt all less than 100 percent, the only healthy safeties on the 49ers roster are Lorenzo Jerome and Adrian Colbert.

Reid, 25, played in 47 of 48 regular season games for the 49ers from 2013-2015, his first three years in the NFL. He made the Pro Bowl in 2013 when he was named to the NFL All-Rookie Team.

Reid missed the last six games of the 2016 season with a torn right biceps. That gave more playing time to Tartt, who started six games and finished the season with 36 tackles, three pass deflections and one interception.

Reid is in the final season of his contract, as the 49ers last spring picked up his $5.676 million option for 2017.

• Quarterback Brian Hoyer only attempted two passes longer than 10 yards Sunday. Shanahan said there were opportunities for longer passes against the Seahawks, but that the protection wasn’t there.

“If you sit back and try to make a living in Seattle versus that pass rush of dropping back deep and just trying to throw deep balls, they are going to lead to a lot more sacks and turnovers than you are deep plays,” he said. “Our goal is to win the game. We tried to call a game and put a game plan together that gave us a chance to win the game at the end. We didn’t think going deep all the time was the best way to do that.”

For the season, Hoyer is 2 for 10 for 26 yards and one interception on pass attempts longer than 10 yards.

• Thursday’s game might be an opportunity for Carlos Hyde to build on the momentum of his performance Sunday when he ran for 124 yards on 15 carries.

Washington rushed for 229 yards and two touchdowns on 39 carries against the Rams in its 27-20 win at the Los Angeles Coliseum. The Rams have allowed and average of 152 yards rushing in two games, third worst in the NFL. Hyde is fourth in the league with 169 yards on the ground.

“I think Carlos runs as hard as he can, breaks every tackle that he can,”Shanahan said. “I think he’s done a great job in these two games.”

• The 49ers signed tackle Pace Murphy to their practice squad. Murphy, 23, appeared in two games as a rookie for the Rams last season and was waived Sept. 2.

Staff writer Cam Inman contributed to this story.