I do believe he has improved in 2 things (better heading technique and more goal sense, for ex. the goal he made vs Novara, at 18 he failed it vs Atalanta), it is not that he has regressed but that he hasn't improved 1 bit in anything else, and those 2 things he has improved well its almost basic requirement that he would've improved in something.
I still see the same erratic player from the U-20 WC in 2007, the same non smart tactical player from the 08 Olympics, the same inconsistent player under Carlo...with the added minus that at least back when he was 18-20 you could see how fired up he came on the pitch even when he sucked (which in the past 2 years is something we have just seen for a 2 months period, and mostly in 2-3 games in that 2 months period, after the derby even if he kept scoring you could see the "everything is cool now don't have to put much into it" attitude).
Seriously, when he was left out of the 2010 WC, it was all Dunga's fault, when Pippo was starting ahead of him with Carlo, it was all Carlo's and Pippo's fault, now it's Ibra's and Allegri's, it's the lack of midfield, his injuries are all Milan Lab's fault, it's never him...at one point you guys have to realize IT IS HIM, his poor physical condition, his consistent tactical errors, his poor form in general, his stalled improvement it is him.
It is pretty telling when the whole team is starting to get tired of him (case in point look at Ambro's and Allegri's reaction when he left the field directly to the dressing rooms in Coppa Italia), and when his GM that has protected him was willing to sell him for 28 mil + 7 in bonus (and that was the offer from "his mentors", they pretty much know that's his worth).
I can see the potential, I still believe he can let it out, but it won't happen in here, he needs to go to another place, a place where he won't be excused for everything and won't be given special treatment to start a derby by the owner (/dad in law). He needs to go to a place where he realizes not everything will be handed to him on silver platter, he needs to get to a place where someone will light fire up his ass so that he wakes up and fullfills that potential. Milan could've still be the place a year ago, even after the season was over, but we're back at square one (for the 4th season in a row!) with him this season, so no it really isn't here where he'll fullfill it.