Willis and Spikes are listed as questionable on the injury report released by the 49ers on Wednesday.
Willis and Spikes each might have broken right hands, which were in bulky casts when the duo reported to practice Tuesday.
Defensive coordinator Greg Manusky also said both linebackers should play. None of the three elaborated on the severity of the injuries.
Willis, who has a team-high 108 tackles, didn't participate in the workout, while Spikes, who has 91 tackles, was limited. Both said the injuries resulted from a helmet to the hand early in Sunday's victory over the Seattle Seahawks.
"I don't know. It got banged up pretty good," said Willis, who played with a broken bone in his right hand during his rookie season. "Right now, it's still under evaluation. We're going to see."
Said Spikes: "Just right now, it's day to day, just trying to see what we can do to hopefully get ready to play."
Linebacker Navorro Bowman (hamstring) was limited in Wednesday's practice and is questionable for the game. Tight end Nate Byham (shoulder), guard Mike Iupati (shoulder) and running back DeShawn Wynn (ankle) were full participants and are probable. Running back Brian Westbrook practiced after missing Monday and Tuesday with what the team listed as a non-injury absence.