Given the dizzying heights Sir Alex Ferguson led Manchester United to during his 26 years as manager, it speaks volumes that one of his lingering regrets centres on Park Ji-sung.
The former South Korea international spent seven glittering seasons at Old Trafford between 2005 and 2012, lifting eight major honours including the Champions League in 2008.
Park was an important cog in Ferguson's squad and racked up 205 appearances in all competitions, yet he was largely overshadowed by superstar teammates such as Cristiano Ronaldo, Wayne Rooney and Rio Ferdinand.
This is why it is quite so striking Ferguson felt one of the biggest matches of his tenure could have hinged on the tireless, gifted midfielder.
The game in question? The 2011 Champions League final, when United came up against Barcelona once again.
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The Red Devils had fallen foul of Barca and Lionel Messi 's brilliance in the final two years earlier, when the Argentine joined Samuel Eto'o on the scoresheet in a 2-0 victory.
United sought revenge at Wembley but ran into a Blaugrana outfit operating at the peak of their powers under Pep Guardiola, who led his side to a 3-1 victory to clinch his second European crown.
Messi was once again amongst the goals - restoring the Catalan giants' advantage early in the second half - to leave Ferguson wondering if he had failed in his tactical approach to deal with the sublime forward.
The legendary Scot deployed Park on the left flank for the 2011 final rather than a more central role where he could have harried Messi relentlessly - a task he had successfully performed to nullify AC Milan playmaker Andrea Pirlo one season earlier.
"That's where I lost the final against Barcelona at Wembley," Ferguson told SportBible in May 2021.
"I should've changed it at half-time and put Ji-Sung Park on Messi. That was a mistake. I realised that after 10 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."
Ferguson expressed his regret over his misuse of Park and the former player admitted last December he also still thinks about "how much I could have done" to blunt Messi's threat.
Park, who turned 41 on Friday, had already proved just how willing he was to put his own technical ability to one side and man-mark a player for the good of the team.
Not only that, he was also hugely effective at this job, as shown by Pirlo's lavish praise for the Seoul-born star in his autobiography.
"They'd programmed him to stop me. His devotion to the task was almost touching," Pirlo wrote. "Even though he was a famous player, he consented to being used as a guard dog."
So, to this day, Ferguson, Park and United fans may wonder if things would have panned out differently at Wembley nearly 11 years ago had Messi been subject to the "guard dog" treatment.
