
As well as the initial £1million fee, United are reportedly set to reap up to the same amount again dependent on appearances and the possibility of Stoke winning promotion.
Potters manager Tony Pulis told the Sentinel: "Four clubs from the Premiership and Championship were chasing him, so to win the race is a tremendous result for us.
"I think a big factor in our favour was how happy Ryan is here, so the dressing room deserves a big pat on the back. If the spirit in there hadn't been so good, he might have been tempted to go elsewhere.
"He's a big, likeable lad who everyone seems to have taken to since he has been here. Despite coming from Manchester United, there has been no 'big-time Charlie's airs' about him. He's simply got his head down and worked hard with the rest of the lads. He' been a credit to himself and to Manchester United.
"He's still only a lad of 20 who's going to be a bit naive and will make mistakes. But what we have bought is potential because over the next two years he will develop into a top, top defender."