Shawcross left Old Trafford for Stoke in 2008 and has become a key player for the Potters.
“I keep saying to myself ‘why did I sell him?'” Ferguson told the club’s official website.
“He’s a great lad and it’s unfortunate that he was here at a time when Craig Cathcart, Jonny Evans and Gerard Pique were showing great promise, and it was difficult to guarantee him a place in the first team.
“But then, of course, Pique left, Cathcart got a few injuries and we decided to sell him to Blackpool, and only Jonny remained. So Ryan would probably have figured now had he stayed.”