Summary
Poirier was never able to get minutes for the Celtics in his rookie season in '19-'20, playing in less than 100 minutes of non-garbage time, according to Cleaning the Glass. Poirier was traded multiple times in the season before being waived by the Knicks.

Poirier is a bit of a dinosaur in today's NBA, as his main value is as a post big. He isn't athletic enough to hold up defensively, especially out on the perimeter or guarding in space, and his offensive skills are not good enough to offset these deficiencies. I don't expect Poirier to return to the NBA anytime soon, and if he did I would imagine it would be on a non-guaranteed contract given to him by a team willing to take a flyer on the international big.
 
Cap Considerations
Poirier was waived by the Knicks this season, and has yet to be signed by an NBA team. Teams will have multiple ways to sign Poirier as even teams without cap space could possibly use any of the Non-Taxpayer MLE, Taxpayer MLE, or possibly even the Room MLE or Bi-Annual Exception to sign him. However, I don't expect any offers for Poirier to be above the minimum.