The defensive end, who made three sacks against the Green Bay Packers earlier this week, injured his hamstring in Thursday's practice.
Head coach Marc Trestman told reporters: "On a personal level, I feel bad for him if he doesn't play because he really had a good game last week, we all know that, and he was moving in the right direction, so certainly I'm disappointed for him."
"[Rookie David Bass] has athletic ability, he's strong up front, he can move around and Mike [Phair] is working hard to continue to develop him.
"We think he's got some upside and he's certainly going to get an opportunity to play if this thing goes as we think it will."
McClellin has made 22 tackles, 3.5 sacks and three safeties so far this season.