According to the San Francisco Chronicle and The Athletic, Shanahan said that Bosa was practicing with the second-team defense in one-on-one drills when the injury occurred. Bosa was done for the remainder of practice, but he remained on the field with a slight limp.
The 49ers coach didn't provide additional details regarding the severity of the injury.
"I'm not sure the severity of it but I know that was it," Shanahan said.
The 49ers selected Bosa in the first round (No. 2 overall) of the 2019 NFL Draft to strengthen the team's pass-rushing unit. San Francisco ranked 18th in adjusted sack rate last season, according to Football Outsiders.
Bosa played in 29 games with the Ohio State Buckeyes. He had 77 total tackles, 17.5 sacks and 29 tackles for loss during his collegiate career. In his final season at Ohio State, Bosa was limited to three games due to a core muscle injury.