The 36-year-old made a stunning comeback to the club in August but United's struggles this campaign led to the sacking of Ole Gunnar Solskjaer who was replaced by interim manager Ralf Rangnick.
Ronaldo admitted that Rangnick needs time to implement his ideas but said the players must take more responsibility for the lack of consistency.
"I knew that my coming here would be challenging," he said. "My expectations haven't changed much since I arrived, but the truth is that we are not as good as we should be.
"I say that on an individual and collective level as well. I think Manchester could be better in the Premier League, we could be playing better, but we have room to grow. It's a new year. We have to think positively, believe it's going to be a good year.
"We have time to make changes, to believe in the new manager's project. We have a lot of changes in the playing system, we changed a lot of things from Solskjaer to our new coach, and we had two weeks without games because of COVID-19 cases at the club -- that has affected us a little bit. I think we have to think positively, and we have everything in our hands to improve and have a good season."
Ronaldo missed United's unimpressive midweek win over Aston Villa in the FA Cup but is expected to return to action when they face Steven Gerrard's side in the Premier League on Saturday.
"It would be very sad not to make it to the World Cup, but I still believe we will be there," Ronaldo added. In football and life, we go through difficult times, but if we fall we must have the ability to rise.
"I believe that Portugal will play in the World Cup! We know we have one or two difficult battles, we have the match against Turkey, and if we win and Italy also wins.
"I believe! In March we will be ready, it will be a battle. Life is like that, there is no perfect scenario, we have to face things as they are -- and face to face. We have time to enjoy the competitions we have and when March comes we will be ready."