Sir Alex Ferguson believes his decision not to put Ji-Sung Park on Lionel Messi cost Manchester United the 2011 Champions League Final.
Barcelona secured their fourth European Cup with a sensational 3-1 victory at Wembley.
It was their second title in three years under Pep Guardiola as they produced arguably the greatest ever final performance.
But Ferguson, whose United side had also lost to Barcelona in 2009, felt a tactical tweak could've turned the contest.
Messi, who scored to put the Catalans 2-1 ahead in the second-half, was instrumental but the Scot felt he could've limited his influence.
"That's where I lost the final against Barcelona at Wembley," he told SportBible.
"I should've changed it at half-time and put Ji-Sung Park on Messi. That was a mistake. I realised that after ten minutes.
"I was going to do it at half-time, but we'd just equalised before half-time and I thought they might've seen the game differently.
"We were quite good in the last ten minutes of the first-half, we'd come into it and we could've been in front actually.
"But I think if I'd have played Ji-Sung Park against Messi I think we'd have beaten them, I really do.
"Messi had two highlights, a fantastic goal that he scored and then made the other one. He was a great player, but if there was a player to do it (nullify him) it was Ji-Sung Park."
The South Korean was often called upon fir big games and came up with crucial goals during his Old Trafford career.
Ferguson named Ji-Sung Park as one of the most underrated players he managed during his time in charge in Manchester.
Brian McCLair and Ronny Johnsen were also mentioned.
