I'm greatly surprised at the number of people who don't consider him a United legend, I thought this would be unanimous. His 3 seasons from 06-09 were probably the greatest by a player in a United shirt and he managed to win the ballon d'Or once, win the league three straight times, got to 1 semi, 2 finals in Europe, while winning one. That in itself should clear any doubts of his status with the club.
If you need any extra incentive, Neville thought he owed Ronaldo his PL medal for the 2007-08 season in his autobiography, Ferguson has a chapter in his own book dedicated to him and he was one of the few people requested to speak at Ferguson's retirement despite him being at a different club. The supporters still love him and sing his name at games (they even sing Forlan's name but its not the same).
You don't necessarily have to stay at the club for your life to be considered a club legend.