Via Chris Forsberg
According to the team, Perkins officially underwent arthroscopic knee ligament reconstruction with meniscal repair at the New England Baptist Hospital in Boston.
...The Celtics did not disclose a timetable for his return, but Perkins suggested he would be sidelined for five to six months after the procedure, which would peg his return at sometime in January or February of 2011.
So it seems like it's an ACL tear and a meniscus tear. Not pretty. We hope for a speedy recovery, Perk.