It all depends on what position they think he can excel at in the NFL.
I actually think it would be a waste to play him at OLB in a 4-3 but I also don't think he is a 4-3 DE so where would they play him? You don't take a situational pass rusher with the 9th pick
Edit: for the record i was not calling Mack a situational pass rusher, simply stating that i think it would be a waste to draft him if he doesnt have an every down position in a 4-3 defense