The 2009 Champions League final saw the sides meet in Rome, with now-Barca boss Xavi winning Man of the Match and current Manchester City manager Pep Guardiola in charge of the Spanish side.
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Sir Alex Ferguson's Man United, winners a year earlier, were looking to make it back-to-back wins.
Ronaldo - always eager to leave a mark - was unable to join an elite list of players with successive European Cup wins.
He left United that summer, but had to wait until 2017 to accomplish the feat at Real Madrid.
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Going into the game, there had been plenty of talk around whether Messi - who had been in Barca's first team for five years at this point - had more to his game than just scoring with his feet.
At 5ft 7in, there were questions over Messi's aerial threat.
But the Champions League final proved he was pretty good in the air, too.
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With Barca already 1-0 up thanks to an early goal from Samuel Eto'o, they were looking to double their advantage in the second half.
And despite going up against Man United's tall centre-back pairing - 6ft 2in Rio Ferdinand and Nemanja Vidic - Messi, slightly off balance, rose to head Xavi's cross into the back of Edwin van der Sar's net.
In 2017, Messi recalled even he was shocked his effort found the back of the net, given who was marking him.
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"It was hard to imagine that I was going to score with my head with [Rio] Ferdinand standing near me, but I didn't really have a marker - the ball came into the centre and I was there to meet it," he said.
"In the moments that the ball was in the air from Xavi's cross, I pictured scoring this goal and I thank God it happened.
"It was such an important goal in every sense: for the team, for the way the final was turning in our favour and for me, too. It's still one of my favourite goals.
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"After missing the 2006 final through injury, it was very special for me just to play in this game, let alone score in it. It completed a great season when we won everything."
And Ferdinand later admitted he was shocked Messi was able to beat him, too, though he knew how talented that Barca side was.
"In 2009, we thought we were ready for Messi," he said. "We had played against him before and we had locked him down. But this was a different Messi.
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"This was a Messi with more freedom to run around, pop up anywhere and surprise you.
"That one team had four of the best five players, apart from Cristiano Ronaldo they had Messi, Xavi, Andres Iniesta, Sergio Busquets. And they had Pep Guardiola coaching them.
"What I used to do was gamble on their lack of ability to get the ball past me, but my problem was the player they had on the ball was Xavi!"
He then continued: "In 2009 and 2011, we were facing the best team in the world with one of the best players ever to play.
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"I look at that now and think it was so unlucky to then have to play them. Messi never made any eye contact, nothing.
"He just separated himself entirely, playing away from you and then all of a sudden he would turn up. Bang."
Two years later, Barca beat Man United again in the 2011 Champions League final, with Messi scoring again in a 3-1 win, and the Red Devils have lost to them in their last two games too.
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Perhaps Erik ten Hag can change that narrative now Messi is at PSG.