The centre-back has made a name for himself with the Potters since leaving in January 2008.
Ferguson believed he could allow the rugged defender leave due to the emergence of Gerard Pique. But the Spaniard returned to his homeland to join Barcelona that summer.
Ferguson, quoted on United's official website, wrote in his programme notes for Tuesday's clash with Stoke: "It always grieves me when I have to release a young player of great potential. And Shawcross certainly fell into that category.
"He had the misfortune - though he perhaps doesn't see it that way considering the success he has had with Stoke - of coming through along with a number of other young defenders.
"This was the era of Gerard Pique and we kept him at Ryan's expense, only I didn't know that Gerard was growing homesick and, soon after, he was telling us he wanted to go back to Spain.
"He has prospered with Barcelona, as we know, and, if I had known he wanted away then, we would have kept Ryan Shawcross."
