Not every "great" player (and he was a promising kid, not "great") can cope with the pace and intensity of the Premier League. At the end of the day, the Portugese league is a lower standard than the PL, so it's quite possible for a young player to thrive in Portugal, yet struggle in England, for reasons that have nothing to do with his position/role in the team.
Look at how long Nani took to really thrive at United? He's older than Anderson, was signed at the same time as him and only really came good just over 12 months ago. He also seems to be someone who puts a lot more effort in terms of diet/exercise/lifestyle - when you compare their physiques - and has been a lot luckier with injuries (which may be related to my previous point)
I still think Anderson will come good. He's already put in some fantastic performances this season, which are hopefully a glimpse of the future. He just needs to look at Nani for inspiration and match his hard work and patience. If he can do that, then he really will deserve to be called a great player one day.