Newcastle's Italian defender, 21, joined from Inter Milan for £5.3million a year ago - but struggled to settle on Tyneside.
But boss Alan Pardew, who plans to play Santon at Old Trafford tonight, said: "Davide looks to be in a good place right now and I'm really pleased with him.
"His language skills have improved and he has a North-East girlfriend cooking him some nice pies, so he's a bit happier.
"I'm so pleased for him. He's a lovely lad, he loves the area and does a great job for us.
"He's really growing into his role and is very effective for us - he's been a revelation so far this season.
"He was still a boy last year but he's starting to play like a man.
"His raids down the left-hand side are becoming more potent. He still has lots to learn to be a top Italian full-back but he's making some inroads."