BOSTON — Brad Stevens, Al Horford, Jayson Tatum and the Celtics’ watched as Kristaps Porziņģis played through a 32-minute scrimmage one week ago opposite of Xavier Tillman Sr., who volunteered to help him out. After he teared through the Maine Celtics, looking like Porziņģis, Joe Mazzulla learned he’d have his starting center back in one week. More than one month earlier than expected.
