With the signing of Al Horford, which was officially announced on Friday, the Boston Celtics have already improved significantly since being eliminated by the Atlanta Hawks in the first round of the playoffs. Aside from the Golden State Warriors, no team added a better free agent than Boston did. Celtics general manager Danny Ainge, though, does not consider his offseason to be over.

From ESPN's Chris Forsberg:

"I think we're not done," Ainge said while clutching his iPhone in one hand, seemingly waiting for the next call that might lead to a deal.

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Asked what he might seek for his team before finalizing roster construction, Ainge said: "We need more shooting. We have some guys who are versatile and can play center and 4 [power forward]. We can use a stronger center, and those are the two biggest things. Maybe another ball-handler as well."

Ainge said this from summer league in Utah, where guard Terry Rozier made a game-winning 3-pointer on Thursday. Rozier has shown flashes that he could be ready for a regular role in the rotation next year, and guard James Young shot 8-for-12 from 3-point range in three games. Both are recent first-round picks, and they are joined on the summer league team by five other players -- Jaylen Brown, R.J. Hunter, Guerschon Yabusele, Jordan Mickey and Demetrius Jackson -- who could be on Boston's roster next season. Or they could be trade bait.

The Celtics are in a rare position. They won 48 games last year, then added a star player and selected Brown with the No. 3 pick in the draft. Ainge's patient rebuilding process over the last three years means that the front office still has a collection of young players and draft picks he can use in trades to try to make the team better. He could strike at any time.

Even if Boston does nothing of consequence in between now and training camp, it could win 50-plus games next year. This should be one of the league's best defensive teams, and Horford should make scoring easier for everybody. If Ainge does make more moves, he is dealing from a position of strength.