The 20-year-old is being tracked heavily by both United and Chelsea.
"He's in a position of strength at the moment," Mills told talkSPORT. "He wants to go and win things, that's the most important thing.
"He's going from a fantastic club, he's gone through his development phase, he's now going to start hitting the peak development phase and he wants that to be the best years.
"There's no point going to Manchester United next season if they're going to finish sixth or seventh."