"It doesn't look good," Rivers said after practice on Monday. "He couldn't get through even the walkthrough portion."
Allen had some success against Bryant in the regular season, but the Celtics guard injured his Achilles tendon in a post-practice pickup game during the Eastern Conference finals and hasn't played since.
Allen started the regular-season game against the Lakers in Boston on Dec. 30 -- one of just 11 games he started this year -- and helped hold the NBA MVP to 22 points on 6-for-25 shooting.
"He's played good in two games against the Lakers," Rivers said. "But if he's not healthy, he's not healthy."