McLeish was resigned to losing Young to United shortly after he replaced Gerard Houllier in June and is not surprised at how quickly the player has made an impact with the Red Devils.
He said: "Ashley is a great player. Reading his story was fascinating about how he was discarded as a youngster and had the determination to make himself a top pro.
"It's a brilliant story that any youngster should read. He's been a real success story. He's a dangerous player and going to United has not affected him.
"He's not fazed by it and he's almost taken up the gauntlet in showing the other United players how good he is."
McLeish added: "Ashley staying here was never going to happen. I knew that from day one after I came here.
"The club tried very hard but it's difficult when one of the top clubs in the world comes in.
"If there was any chance, I would have tried because you don't want to lose the top players."