Since United spent £89m on him they have failed to mount a title charge and picked up just three trophies in five seasons.
Keane told Sky Sports: "I like Paul as a player, simple as that. From what I hear, he's a pretty decent human being, a good character.
"Slightly different in the way he leads his life, but he's a talent boy.
"Again the one thing I'm disappointed with is, I don't think he's made Man Utd that much better or the players around him better."
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