The striker, who netted 150 United goals, insists he 'holds no grudges' against Ronaldo.
Talking in the latest edition of Inside United magazine, Van Nistelrooy said: "My final months were difficult but I don't think of those.
"Ronaldo's improvement has been incredible since then. He is on a par with Kaka and Lionel Messi.
"But if he carries on like this, he'll be the best. He has got everything."