Age difference is significant in that comparison. LaPorta debuted at 24, Montero is still only 23 and has over a full season (albeit a disappointing one) under his belt. He hit .295/.330/.438 away from Safeco last year, so there's still some hope for a resurgence. Approach needs a ton of work. Getting out from behind the plate may help.
Honest question since I don't see Montero hit often. What's wrong with his approach? He was never a high walk guy in the minors, but his walks have dropped in the majors. Is that what you are referring to? Is he getting tied up inside, perhaps? He always had some nice power to right, but perhaps everyone is pounding him in with the fastball and then getting him to chase off speed down and away? As I said, just curious.