Ive always had the idea that if Ronaldo was the one playing all these years for Barcelona and Messi for Madrid, then Ronaldo would have been praised as the best in history and obviously above Messi.
But saying Messi couldn't do as well in any other team is a little risky, We dont really know how he would do in another team but he has played really well in Argentina and he is the one that other players depend on in Barcelona so.. I dont know why Sir Alex said that.
"Messi is Barcelona player " - I choose to agree very much with this statement .