The two forwards have not clicked so far this season under the management of Ole Gunnar Solskjaer.
While both have scored important goals for the club so far this season, Greenwood and Ronaldo are often not on the same wavelength when the team attacks.
According to ESPN, some of the United players are worried that Solskjaer is not getting the best out of the duo.
Ronaldo is frustrated at Greenwood's poor decision making, with the youngster often choosing to shoot instead of passing to an open teammate.
The Portuguese has reportedly become 'visibly irritated' with the United academy graduate.
The revelation comes after United were thrashed 5-0 by rivals Liverpool at Old Trafford on Sunday.
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