BOSTON (AP) Jim O'Brien quit as coach of the Boston Celtics on Tuesday after half a season under new boss Danny Ainge, a team source told The Associated Press.

The team said it would have a ``major'' announcement but did not say why. The source said on condition of anonymity that the team would announce O'Brien's departure.

The Celtics are 22-24 after reaching the playoffs the past two seasons.

O'Brien's resignation came one day after Byron Scott was fired as coach of the New Jersey Nets, the only team ahead of Boston in the Atlantic Division.

The Celtics lost five of their last seven games, including a 110-91 setback at New Jersey on Sunday. Boston trailed by 17 at halftime and sank just 36 percent of its shots for the game. In those last five losses, Pierce, who hasn't gotten much offensive help, made just 27 percent of his shots.

Since taking over, Ainge has made numerous changes, trading Antoine Walker to Dallas before the season and Eric Williams and Tony Battie to Cleveland on Dec. 15. The first deal deprived the Celtics of a second scoring threat to go along with Paul Pierce. The second took away two veteran leaders and keys to O'Brien's defense-oriented approach.

Ainge, hired May 9, gave O'Brien a two-year contract extension running through the 2005-06 season. Before granting the extension, however, Ainge met with O'Brien and considered potential replacements among current college coaches, pro assistants, retired players and former NBA coaches.

O'Brien became coach when Rick Pitino resigned in January 2001. He took the Celtics to the Eastern Conference finals in 2002 and the conference semifinals in 2003. They lost both times to New Jersey.

There was no immediate word on who would succeed O'Brien.

The Celtics' next game is at home on Wednesday night against the Detroit Pistons, who are second in the Central Division with a 29-16 record.