Terrence Williams joined the Celtics for their morning shootaround here Tuesday, but he's currently awaiting a letter of clearance from his former team in China before he can begin playing with the Celtics. Until that process is completed, Williams, whom the Celtics signed to a 10-day contract Monday, cannot participate in basketball activities with his new team. "Sometimes that’s quick, sometimes that’s a long process," said Celtics president of basketball operations Danny Ainge. "We’re trying to finalize that paperwork. I’m hopeful that it will get done soon." The Celtics have 13 players on their active roster, one over the league minimum, but three of them have suffered season-ending injuries. Williams, a 6-foot-6-inch wing player who has played with three NBA teams, will be a vital addition, if only because he will help the team more evenly distribute minutes.