Associated Press For the second time in less than a week, Boston Celtics coach Doc Rivers had to tell his team that they would be without a key contributor for the remainder of the season. The Celtics announced Friday that first-round draft choice Jared Sullinger underwent successful lumbar disk surgery and will miss the rest of the season. He is expected to be ready in time for next season’s training camp. “He was playing great,” Rivers said. “I think the good news is we know he can play and we know he will be a very good player. In the long run, this will make him healthier.” The announcement came five days after All-Star point guard Rajon Rondo was diagnosed with a torn ACL that will keep him out for the rest of the season. “Losing Rondo was big enough for us and now we’re losing Sully, one of the best rebounders on the team,” guard Courtney Lee said. “It was definitely shocking. We didn’t know how serious [his] injury was when he left the game.” Rivers told his team after Friday morning’s shootaround that Sullinger would be out for the season.