The Englishman, who enjoyed a decent second season at the Red Devils after a sensational first campaign, was asked about the two greatest players in footballing history.
Greenwood, who may have grown up idolising Ronaldo given the Portuguese star played for United earlier in his career, believes that Messi is the superior of the two.
In a Q&A with MUTV, Greenwood said: "For me, it's always got to be Messi. For me, he's just on another planet.
"Ronaldo is obviously amazing, a physical, great athletic footballer.
"But to do what Messi does at his size, what he's done for Barcelona and winning that trophy for Argentina, he is just on another level."
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