I'm not touching the value of the trade because both fan base's have repeatedly said no, but it would be a terrible move by Mtl to trade a top D prospect (that was badly needed) for Drouin and then immediately trade him the next year when his value has fallen.
Play Drouin on the wing where he belongs and get a good center to play with him, don't trade him when his value is at the lowest... If MB trades Drouin for anything that can be seen as a downgrade (relax I'm not saying RNH is a downgrade, I'm talking in general) it would mean he made a mistake, which he won't do, and I don't think many teams are lining up to overpay for him.
That being said, Drouin has plenty of skill and is fast as hell, he would look amazing on McDavids wing.