Pierce went through drills on Monday, but did not participate in the team's practice Tuesday after experiencing soreness.
"I'm concerned," Doc Rivers told reporters after the practice. "I'm not concerned long-term, but I'm concerned short-term."
Losing Pierce for any extended period of time would hurt the Celtics, whose depth at small forward is not a team strength since losing Jeff Green for the season. If Pierce misses any regular season games, Marquis Daniels would likely start in his place and Sasha Pavlovic would see an expanded role off of Boston's bench. Rivers said it's also a possibility for Daniels to remain with the second unit even if Pierce cannot suit up, because Daniels has fit so well with the bench during practices.
Pierce won't play in Boston's final preseason game on Wednesday against the Toronto Raptors. Rivers said the plan is for Pierce to practice on Friday in hopes of being available on Sunday when the Celtics open the season at Madison Square Garden.