What’s The Call: Would Morgan Rielly have gotten a penalty?

What's the call on Morgan Rielly surrounding the helmet dilemma?

Late in the third period of a game between the Toronto Maple Leafs and Boston Bruins, Morgan Rielly takes a hit and loses his helmet.

Rielly goes to the Leafs bench and waits for a new helmet. He is unable to get one that fits before the Bruins score to take the lead.

If Rielly had come back on the ice without a helmet what would have been the call?