Shawcross will skipper Stoke against his former club at the Britannia Stadium but he could well have been starting for the opposition had Ferguson not agreed to sell him in 2008.
Shawcross had impressed on loan for the Potters but decided to make the move permanent due to the competition for places at Old Trafford.
The defender left United without ever making a Premier League appearance but Ferguson admits he would be involved more regularly now had he stayed.
"I keep saying to myself 'why did I sell him?'" Ferguson said of the 23-year-old on 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."
Shawcross' Stoke have once again found themselves in the spotlight for a perceived over-physical approach following comments made by Danny Murphy recently, but Ferguson believes United are facing one of the most fair and honest sides around.
"They've gradually improved every season in the Premier League and Tony (Pulis) is a real good guy, with great principles.
"His team is fair and honest, very tough, and there's no quarter asked or given. We're playing one of the most honest sides you can meet in the Premier League."
Ferguson is also aware that his side will have to be at the top of their game to maintain their 100% against the Potters since their promotion back to the top flight.
He added: "Tony will have his team galvanised for this game. They have the noisiest support in the country, so we'll need control that and keep them as quiet as we possibly can.
"We have a good record there. We've had to adapt our game when we've been there in the last couple of years, and it'll be the same on Sunday. They put you to the test alright, but hopefully we can cope with that."
Wayne Rooney will not be involved at the Britannia Stadium but Rio Ferdinand is one of a number of first-team stars set to return following rests in midweek.