Campbell admits at times he was in awe just training with United's stars.
He said: "It can be hard for young lads there.
"I'm a United fan so it was awesome to be there but it may have been better if I wasn't - I would be watching too much in training instead of playing.
"It was class training with the likes of Wayne Rooney. United have great footballers and some of the best players in England, so training with them was what you dreamed of when you were younger."
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