The Stoke City skipper, who returned to Old Trafford on Tuesday night for the Barclays Premier League clash with United, and has forged a reputation as one of the most reliable centre-backs in the division.
A permanent deal with the Potters was agreed by the boss back in January, 2008 due to the potential of Pique, only for the Spaniard to move back to his homeland later that year to enjoy great success with club and country.
"It always grieves me when I have to release a young player of great potential," Sir Alex wrote in his programme notes for United Review. "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."
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