The San Francisco 49ers' third-string quarterback, C.J. Beathard, could find himself active for the first time this season on Sunday against the Washington Redskins. Of course, that is all dependent on the availability of another player.

No, not another quarterback.

It's dependent on Raheem Mostert's (knee) game-day status. The running back practiced for the first time this week on Friday and is listed as "questionable" for Sunday.

Head coach Kyle Shanahan explained on Friday why Beathard might be active if Mostert can't play.

"Only two guys usually dress on game day," Shanahan said. "C.J. will dress this week if Raheem can't go because he'd be the last guy up. And if we got another guy sick or something in the next two days, we'd probably go with 45 (players on game day) because we don't see someone we want to cut."

Much of that has to do with the fact that the 49ers are carrying several injured players on their active roster, all of who will be inactive on game days. San Francisco didn't want to potentially lose them for the season to injured reserve, so they remained on the active roster. Those names include tackles Joe Staley and Mike McGlinchey, cornerback Ahkello Witherspoon, and fullback Kyle Juszczyk. Each is a starter, and each is expected to return later in the season.

Shanahan was also asked about the potential of moving one of his three quarterbacks as the league's trade deadline quickly approaches, and whether or not he discusses the possibility with his signal-callers.

"I think they're pretty smart," Shanahan responded. "It's something you mention at the beginning of the year to them, that they understand. I think everyone knows when you do keep three quarterbacks on the roster, that's always a possibility. But they also know there are three guys that it would be hard for us to let them go. We do want all of them here."

Added Shanahan: "You talk to them about it, but after that, it's out of their control. They've just got to work as much as they can, and if that time comes, it is what it is."