Of course, we don't know whether Warner tested positive for the highly contagious virus or if he came in contact with someone who did. And NFL teams are not permitted to comment on a player's medical situation concerning COVID-19 other than referring to the roster status.
For the 49ers, who might have started to feel safe given all of the precautions the team has put into place, it is a brutal reminder that COVID-19 can unexpectedly impact a roster at any point.
"I think it's just a good reminder that no one's out of the woods," Shanahan told reporters on Monday. "When you go a few weeks with nothing happening, you start to feel pretty safe. It's a reminder that this stuff is out there.
"We'll see how it goes. I'm not going to get into it because I can't, but you guys know the rules with this. There's a couple of options with the tests that come back, and we'll see how that goes each day."
Shanahan didn't immediately have any answers on who might take over relaying the play calls to the defense in Warner's absence, should the linebacker miss extended time. The team does have two weeks' worth of practices to figure that out, though, if necessary.
Defensive lineman Arik Armstead was asked about what this reminder means to him.
"I think the No. 1 thing is, hopefully, Fred is OK," Armstead responded. "But it's just a reminder of the seriousness of what's going on. I know you get into the groove of things, you get in a rhythm, you get in a schedule, and you kind forget about COVID and how serious it is.
"An incident like this — hoping Fred is OK — but it's a quick reminder of how things can go, and how we need to continue to be safe, take care of each other, protect each other, protect our family, continue to practice safe and healthy habits, social distancing.
"It's definitely a wake-up call and a reminder for all of us that are just getting into a rhythm of going through camp of how important it is to continue to be on top of those things."
In the meantime, Warner's presence on the defense will be missed.
"Fred gets better each year," Shanahan said. "He came in and played at a high level as a rookie. We put a lot of pressure on him as a rookie, and that pressure paid off because it gave him a ton of experience. He rose to the challenge then, and he's taken it to a whole other level each year.
"Fred is as much of a pro as I've been around. While he's not in here, I'm sure he's in his apartment doing bag drills, or wherever he's living, just going crazy that he can't be in here. So, Fred's a stud. Whenever he can come back, he will be ready to go."
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