I would have started Matthew in Providence to heal the shoulder fully and test it in game action.
Also, shake off rust; continue to learn how to play against men who are physically stronger and bigger; and work on getting rid of the puck before a hit, especially a late hit, even when his instinct is to hold on to it in service of making a play.
There it would have been no shame in that, and if he needs to go down to PRO for those reasons going forward, there's no shame in that either. He's 20 years old.