Celtics.com is reporting that Pierce will return to Boston and will not play tonight due to an infected finger. He is listed as day to day.
Wally is listed as starting at Shooting Guard and Gerald Green will take Pierce's place in the starting lineup. This is probably similar to most preseason situations. I imagine that if it was a game that counted, Pierce would be out there but there isn't a reason to force things when the kids could use the minutes and the game doesn't count as far as win/loss record.
UPDATE: The Globe has an update on this as well and their article mentions the fear of infections spreading through the team.
The severity of the infection is unknown, but the Celtics are clearly taking it seriously by sending Pierce back to Boston. That said, the early return to Boston is precautionary than anything else, though Pierce is experiencing discomfort in the finger and has difficulty bending it. Team officials want to make sure Pierce is completely healthy when games count. Also, in light of Delonte West's toe infection, the Celtics also wanted to make sure that infections do not spread throughout the team.