Man United have lost four of their last six league games, recording just one win in the process, while they have been comfortably beaten by Liverpool and Manchester City in their last two home domestic fixtures.
According to the Daily Mail, Ronaldo has become alarmed at the drop in standards at the club since he left 12 years ago for Real Madrid.
The report claims that the Portugal international is 'shocked' at how much things have changed since his first spell between 2003 and 2009, and the 36-year-old is becoming increasingly frustrated amid the team's poor form.
Ronaldo had Man United's only shot on target in the Manchester derby on Saturday, with the 20-time English champions spending the majority of the match without the ball.
The Red Devils will be looking to return to winning ways in the league when they make the trip to Watford after the international break. body check tags ::