The rookie linebacker has a mild AC joint sprain in the same shoulder that required surgery following the 2016 college football season.
Foster and the shoulder have both looked fine so far in training camp - he hasn't missed a day of practice - but an issue that sprung up on Thursday will keep him away from contact in the 49ers' joint practice with the Broncos.
San Francisco also got nice performances from a couple of guys hoping to make the team, running back Raheem Mostert (15 carries, 89 yards) and Kendrick Bourne (four catches, 88 yards and a TD).
Foster was viewed as a top-10 talent, but he slipped to No. 31 overall partly because of concerns about his shoulder.
"I talked to him and like Reuben, big smile, said, 'I'm straight, '" Lynch said.
Foster's injury is among several to strike the 49ers linebacker corps, which picked up a reinforcement Thursday with the return of Shayne Skov, a Stanford product who's had an on-again, off-again tenure with the club since 2014. "He's good", when asked if Foster had sustained a serious injury. What we do know now is that C.J. Beathard is going to be second on the field against the Broncos while Matt Barkley will be the third signal caller to enter the game. "You just try to be out there because you love the football game". He definitely wanted to go.
Shanahan noted Foster's injury is common among linebackers. They will now battle the Denver Broncos in their next exhibition affair on Saturday evening. Malcolm Smith (pectoral) went on injured reserve in the first week, and Wednesday the 49ers waived/injured Donavin Newsom (concussion) and Jayson DiManche (hamstring).