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."(It was) one of those moments where it's, `Man, it can't get much worse than this,'" said Lee, who was unaware Sullinger had the surgery. "We want to stay positive. We need to continue to fight and continue to push."For the Celtics, ranked 29th in the league in rebounds entering Friday night's game, the injury means others will need to contribute on the glass. In 45 games, Sullinger averaged six points and 5.9 rebounds, which was third-best on the team."We will have to play differently - again," Rivers said. "We will definitely have to be a small-ball team. We'll have to be creative. Our perimeter guys are going to have to rebound more. The guy I will put the most pressure on, and it's a little unfair because he's never been a great rebounder, is Jeff (Green). When he's in the game, he will have to rebound."Sullinger had been tabbed as a lottery pick before health concerns dropped him to 21st overall in the draft."We're very happy to have drafted him," Rivers said. "We felt that this would happen at some point, but we just thought it would be during the summer where we could just say it is time to do it. It just happened now."