United brought in the likes of Ronaldo, forward Jadon Sancho and centre back Raphael Varane in the close season but have made a stuttering start domestically to sit sixth in the table with 14 points from eight games.
"The adaptation will take time, even the system of the game that we play, but I think step by step we have to put it in our mind that everything is possible," Ronaldo told Sky Sports.
"I don't speak only about my individual stuff, I put the collective in first place. To win things as a collective - it's more easy to win individual stuff - I still think it's possible."
Ronaldo has made a strong start to his second spell at the club, scoring six goals in all competitions so far. The 36-year-old called for "sacrifice" from his colleagues to lift the team.
"Everyone should know their role. I know my role in the team, in the club... My role is to score goals, to help the team with my experience and know-how to understand the game," the Portuguese added.
"If everyone is thinking like that, sacrifice for the team, I think we will be a better team. We have fantastic supporters behind us, fantastic stadium, fantastic team, so we have to carry on like that."
United take on bitter rivals Liverpool in the league at 9:30 pm BDT on Sunday.