The NBA-leading Boston Celtics added some depth at center on Wednesday, acquiring big man Xavier Tillman from the Grizzlies in exchange for forward Lamar Stevens and two future second-round draft picks. With the deal Boston adds some depth in the frontcourt behind 7-footer Kristaps Porzingis, who has struggled with durability, and 37-year-old veteran Al Horford. The 6-foot-8 Tillman has appeared in 34 games for Memphis this season, averaging six points, 4.6 rebounds and a career-best 1.7 assists while shooting 40.8% from the field.