Fletcher feels his former United teammate Ashley Young now looks a natural at left-back.
He told talkSPORT: "Everyone knows (Jose) Mourinho likes his defenders to defend first and foremost and anything you can give the other way is a real bonus and Luke Shaw looked like he was tying into that the other night. I thought his performance was fantastic, not only going forward but defensively.
“It is all character building stuff for him but because he is a young lad this might be the making of him in some ways.
“People might say Ashley Young is not a left-back, he is right-footed, he used to be a winger, but plenty of great left-backs have been right-footed.
“Youngy is getting a little older now so he might see that as his long-term future. That is probably going to be Luke Shaw's biggest challenge more than anything."