ESPN's Adam Schefter first reported that the star pass rusher suffered a torn ACL in Sunday's 16-15 win over the Arizona Cardinals. Bosa will undergo surgery and miss the remainder of the season.
"I got to talk to Nick last night," Shanahan said. "It wasn't confirmed until this morning. I haven't been able to see him yet today, but I talked to him last night. He had a pretty good idea what we were going to find today, but obviously, he's bummed out. But Nick can handle anything, and he's going to handle it like he always does, which will be great.
"But obviously, he was bummed out from it, not being able to finish out this year, and especially [with] how good of a year he was having so far."
Shanahan emphasized the challenge of moving forward without Bosa.
"You don't just replace a guy like Nick," Shanahan said. "Everyone's got to continue to get better at everything. The same thing I'd be preaching if we did have Nick, too. When you lose your better players, it gets harder, but I also expect, throughout this year, all of our players to get better as it goes, and that's stressed that much more when you lose a great one like Nick.
"Lots of guys, obviously, will have to step it up as it goes, but I believe that'll be more by committee than any individual."
Shanahan added that Bosa knew the injury was serious on Sunday, and an MRI on Monday confirmed it.
As for the severity of the injury, Shanahan explained, "I was told it was as clean as it gets. So, in terms of if you have an ACL, I was told that it was the kind that you want."
This setback is the second ACL injury of Bosa's NFL career. He tore his left ACL in 2020, while this latest injury is to his right knee.
Shanahan also provided the following additional injury updates:
- DT CJ West suffered a broken thumb and will undergo surgery on Tuesday. The team will update his status later in the week.
- QB Mac Jones is day-to-day with a knee injury.
- DL Mykel Williams is day-to-day with a wrist injury.
- CB Renardo Green is day-to-day with a neck injury.
On West's situation, Shanahan added: "He'll have a chance, they say. We'll see how it goes this week, but he'll have a chance at the end of the week with a cast. And if not, he should be good the following week. But we've got to see how this week goes with him."
Brock Purdy update
Shanahan expects quarterback Brock Purdy to practice this week, though it's uncertain if he'll be limited like last week or a full participant.
"I don't know whether he'll have limitations or not, but I expect him to get back out there and get some stuff in, and we'll see on Wednesday how much he's able to do," Shanahan said.
The 49ers hope Purdy can return to the starting lineup on Sunday against the Jacksonville Jaguars after missing the last two games.
Wide receiver updates
Shanahan expressed excitement about wide receiver Demarcus Robinson returning from suspension. The team will evaluate his physical readiness when this week's practices begin on Wednesday.
The 49ers were without Jauan Jennings in Week 3, as the receiver deals with ankle and shoulder injuries. While there is no definitive timeline for his return, it doesn't sound like he is dealing with a long-term setback.
"And then J.J., same boat as last week," Shanahan said. "We'll evaluate him as this week goes. I guess we'd call that day-to-day with J.J."
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