The former United midfielder said: "To triumph at the club is not easy, even if you are a superstar.
"Some players leave because of the enormous and constant pressure. Carlos will find that United is not like West Ham in any aspect. The change will be enormous and if things do not go well he could suffer a mortal fall in fortune."
He admitted: "The people at Old Trafford are very critical. If a player has good fortune from the very first day everyone is positive. But if problems appear it is not so easy to triumph. The only possible goal there is success. Second place is failure and this burns deep into every player and can affect you.
"During the Copa America I spoke several times with Carlos about playing at Old Trafford and told him that while he turned himself into an idol at West Ham, it will be very difficult to do the same at United.
"English football is not as easy for Argentinian players.
"Last season Tevez exploited his situation after a number of months in the Premier League but he still needs to play and do it more regularly.
"Carlos will need help from Sir Alex Ferguson from the first moment.
"He must not be asked to perform wonders until he is well used to training methods and tactics."