AC Milan scored 4 goals in 2 games vs Barcelona recently, that is one of the best teams in football history (we're gonna realise it when we'll be old) with Messi one of the best players in football history, despite this Milan is not one of the best Milan in history.
Barcelona played better (but Milan on similar levels) but they had a result thanks to a strange penalty and an own goal by Milan.
Messi scored his FIRST goal against an Italian team only last November... (only on that strange penalty conceded, that he even had to repeat) and he had to get a red card for a bad foul, 'cause he had already had a yellow one. He was very nervous...
Of course he's a great player, but if he played in Italy, without a team playing for him, he would score less goals and should be careful with injuries, as defences are better