Celtics center Robert Williams expects to be back on the court by Christmas Day, ESPN’s Adrian Wojnarowski said tonight on the network’s pre-game show (video link), relaying a conversation with Williams’ agent. Williams is “progressing well” with his rehab after undergoing an arthroscopic procedure on his left knee in September, Woj adds, noting that it was the second knee operation for Williams this year. Williams was originally projected to miss eight to 12 weeks, and the Celtics have been cautious about his return. He began participating in three-on-three drills a few days ago. Williams became a full-time starter last season…