The La Liga outfit obviously won't say it publicly but it's claimed they've expressed their preference ahead of Friday's Champions League draw.
Messi, whose brilliance was key to Guardiola's two Champions League triumphs with Barcelona, is just as risky as Ronaldo and it will be intriguing to see whether any of England's four clubs can find a way to overcome the game's two modern greats.
The arrival of Ole Gunnar Solskjaer has rejuvenated the Red Devils, after Jose Mourinho's controversial spell in Manchester.
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Former Blaugrana coach Pep Guardiola's team are one of the favourites to win the title and Manchester City have the quality between the four lines, as well as on the bench, to reach the Estadio Wanda Metropolitano final.
United have faced Juventus 14 times in Europe in total.
"I wouldn't like Juventus against Barcelona because I think it would be something really special".
James Milner wants Liverpool to avoid English opponents in the Champions League quarter-final draw on Friday.
"They would all need a period to psychologically switch off and we'd have to look at Champions League final players when we got to it". He helped Real win four of the past five Champions Leagues and it is hard to imagine the Spanish giants winning any of them without the Portuguese star's contribution.
The Premier League might be awakening again as a European force, but Ronaldo and Messi remain the kings of the competition and they should strike fear into the hearts of every English club still dreaming of winning this year's Champions League.