United were 2-0 ahead against the Bees when interim boss Ralf Rangnick made the decision to replace the Portuguese superstar with just under 20 minutes remaining.
However, television footage broadcast shortly afterwards showed the veteran expressing his disappointment at being withdrawn before full time.
The 36-year-old was seemingly shown to say to someone else, presumably Rangnick, on the bench: "Why me? Why me? Why would you take me off?"
Ronaldo had missed the previous two games due to a hip injury, Rangnick alluding to that fact during his post-match media commitments.
United are back in action when they play host to West Ham United in a crucial Premier League game at Old Trafford on Saturday afternoon. body check tags ::