The plus:
Drouin is young, highly skilled & adds a dimension to the Habs which they currently lack.  If they play him at center, & Julien can get him to excel at both ends of the ice, the deal could turnout well especially since the CAP hit is ok. 

If Galchenyuk resigns for reasonable amount, & play him at C, the Habs could have their 1-2 up the middle longterm.

The negative:
Blueline is aging, & they traded a big piece who might've been able to play immediately in October.  The Habs now have to go into FA or trade route to augment the D.  This may mean moving AG for a Dman, which will negate to a great extent Drouin's acquisition, and still leave the team with a lot of work to do up the middle.

In CAP era, drafting & inserting young talent that are on entry level contracts to augment the core is the recipe for success.   If AG is resigned for $6m, the top 8 (w/o Rads)will eat up $43.5m in 2017 & conservatively $50m when Price ($8m) &Max ($7.5) resigned, leaving $25m for 12 players.