The 26-year-old has scored 14 times this season, including a brace against Chesterfield on Tuesday.
Grayson said: "He's our talisman and to have him back gave everybody a boost.
"We haven't rushed him back and it's as if he's never been away. He was moaning at me the other night when I took him off with 10 minutes to go because he wanted to get a hat-trick, and that's the mark of a player.
"I'm sure if Manchester United's players are not aware of him at this moment, come Monday night they will be because he ruffles a few feathers."
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