SANTA CLARA, Calif. -- San Francisco 49ers defensive end Nick Bosa suffered a calf injury in practice Thursday that will cost him a few weeks, sources told ESPN's Adam Schefter and NFL Network's Ian Rapoport on Friday.
The 49ers are leaning toward not placing Bosa on IR in part because they are running out of room for players to be brought back, a source told Schefter.
"It's the discussion," Shanahan said. "[Bosa's] right there at that point along with a couple other guys. They're all in the same boat. Because of the number we have on it, we won't be able to do all of them. We've got to decide and we're just trying to guess which ones will go longer, which ones have the chances of being sooner. It's just a guess. They're all really close to the same boat, so we got to decide that by tomorrow at 1."
Returning to start the season from a torn ACL in his right knee, Bosa had played 70 snaps in the first two weeks combined. He was preparing to play against Arizona in a Week 3 almost exactly a year since he tore the ACL against the Cardinals.
He was listed as a nonparticipant for Wednesday's practice with the designation of "resting player/knee," but the 49ers added the calf to the knee designation Thursday and Bosa did not practice or do his usual weekly media session Wednesday or Thursday.
Shanahan said Bosa tweaked the calf while warming up and doing high knees for the team's walkthrough Thursday morning.
"I know he's bummed out, but he'll be back," Shanahan said
Bosa's injury is the latest and perhaps biggest hit to a ravaged 49ers defensive line. The Niners placed defensive tackle C.J. West (knee) on injured reserve this week and have lost defensive end Sam Okuayinonu (foot) and tackle Alfred Collins (patellar tendon) to IR from injuries suffered in practice the first two weeks.
Rookie defensive tackle James Thompson Jr. is dealing with a high ankle sprain that will cost him three to six weeks, and rookie edge rusher Romello Height has a broken hand. Along with Bosa, the Niners ruled Thompson and Height out for Sunday against the Cardinals on Friday afternoon.
All of those injuries plus a long list of wide receivers dealing with ailments have the Niners already trying to figure out who can go on injured reserve and who can't because they can designate only eight players to come back from that list.
"There's a lot of variables there," Shanahan said Wednesday. "That's why it's not simple just to put everybody on IR return because then eventually you're not going to get one back. We're going to run out of them eventually, so that's what we're trying to balance out, see how those guys are this week.
"It's complicated and we've got to guess."
Without Bosa, the 49ers' healthy and available defensive ends are Keion White, Khalid Kareem and Ogbo Okoronkwo. The team also has Vincent Anthony Jr. and Viliami Fehoko Jr. as options on the practice squad, though the Niners will need to use at least one of their two practice squad elevations on a defensive tackle from the trio of Sebastian Valdez, Kevin Givens and McKinnley Jackson to help fill the void at that position as well.
Another defensive end, Mykel Williams, is on the physically unable to perform list. He is expected to return to practice next week from a torn ACL suffered last season but can't play in a game until Week 5 when the 49ers visit the Seattle Seahawks.
Elsewhere on the injury report, 49ers receiver Mike Evans (hip) is questionable to play against the Cardinals. Evans did not practice Wednesday or Thursday but was a limited participant Friday.