The Scot says recent injuries to some of his key defenders have made him regret allowing the Stoke captain to leave Old Trafford as he is a "quality player"
The defender joined the Red Devils in 2002 as a 15-year-old but left the club in 2008 to join Stoke following a successful loan spell at the Britannia Stadium.
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"We have let a number of our players go there [Stoke] on full transfers like Danny Pugh, Danny Higginbotham, and a player who is currently doing extremely well," Sir Alex explained in the match-day notes.
"Ryan Shawcross is now their captain and I was delighted to see him called up for the England squad because he is a quality player I have wished-particularly of late- was still at Old Trafford.
"We made the right decision keeping Jonny Evans, but I'm not so sure about letting Shawcross join Stoke."
"I just he is not back to make me regret our decision even more this afternoon!
"He is a good type, though, and I wish him well."