I dont think his issue was with Drouin playing with Mack.
The issue is putting Mack and Mikko together. It makes the Avs way too top heavy and leaves the 2nd line much weaker. That trio(Mack/Drouin/Mikko) would also be pretty bad defensively bordering on liabilities.
Mack/Mikko are responsible enough on the defensive side so long as they have a workhorse like Nuke, Landy, or Lehky playing with them. When they don't have any of those guys, they're pretty bad defensively.
And, Drouin certainly isn't a workhorse by any means.
Mack and Drouin can play together all year long IMO and likely do well, but they need one of Lehks or Nuke playing with them for defensive work and also to create time and space for them offensively.