The Celtics had two options when Kyrie Irving "communicated" he wouldn't be re-signing with Celtics. They could spread the cap space around and sign a few good to very good players including a big man, or they could use it all on an all-star to replace the one leaving. Danny Ainge wisely went the latter route. It's much easier to trade for good to very good players than it is for a star, so when you can add a Kemba Walker you prioritize that.
The Celtics quickly filled the big man void by signing Enes Kanter, Vincent Poirier, and re-signing Daniel Theis.