The United star is currently serving a three-match ban for his red card in recent the 1-1 draw with Portsmouth at Fratton Park.
United are without a win in their first three games of their Premier League defence and Ronaldo has now revealed his regret that he is not able to assist his team-mates.
"I'm feeling sad not being able to play," Ronaldo told the Daily Mirror. "It is so frustrating, particularly when we are not able to win right now.
"But I'm optimistic and I have confidence in United, even though the beginning of the season is bad.
"But the situation will change soon and we will be positive."