It involved a selection decision for the 2008 Champions League final, which the Old Trafford club won after beating Premier League rivals Chelsea on penalties in Moscow.
"To win the European Cup is fantastic. It was a really great bunch of players with a great attitude about them and a good, strong squad," the legendary boss told MUTV.
"My problem in the 2008 final, maybe I even regret it to this day, was I left Ji-sung Park out completely in the final.
"He'd played such a great role and that's the problem when you get to these finals.
"At Wembley [in 2011] I did it to Dimitar Berbatov and he took it badly. He didn't deserve it. No player deserves to be left out of the final.
"That's why we try hard at these European seminars with the coaches to try to get 11 subs in the final.
"You've got four or five sitting in the stand who made fantastic contributions in getting there."