Danny Ainge has high expectations for the Celtics this offseason, as the team enters the fourth year of its rebuilding process. Boston showed progress in year three by winning 48 games during the regular season, tied for third-best record in the Eastern Conference. But after a second straight first-round exit from the postseason, the Celtics’ president of basketball operations acknowledged this offseason will be different than recent years. “We look forward to every offseason. This offseason is bigger,” Ainge said Wednesday at his end-of-season press conference. “My expectations are high this offseason, and yet I also know that it takes good fortune. We need the ping-pong balls to bounce our way [at the NBA draft lottery] to give us the best opportunity. Whether we use [the first-round pick acquired from the Nets] or whether we trade that pick, and in free agency, we have opportunities. And that’s all we have.”