Alexander will make it onto somebody's roster, or he'll go overseas for a year or two and try again.
The Celts have a more athletic, shot-blocking version of Alexander in Jordan Mickey. Trouble for a guy like Alexander is that his skill / physical profile is not exactly in demand these days. If you're a PF you need to be able to switch on defense and dominate the boards, or else have the ability to hit outside jumpers and attack a closeout.
He's in the same boat as Thomas Robinson -- talented, but he came into the league at the wrong time and is drastically undervalued as a result.