Cristiano Ronaldo sets Man Utd target and warns he'll leave if they don't hit it
United are currently down in seventh place in the table, 22 points behind leaders City, with any hope of the title long since gone.
Ronaldo admitted United, who lost their last league game 1-0 at home to Wolves, have not played well under interim boss Ralf Rangnick.
But Ronaldo, who returned to United in the summer after 12 years away, believes the current squad have the talent and potential to get back to challenging for the title.
“Let's be honest," said Ronaldo. "The last five or six games, do you think Manchester United played very well? No. You know that, everyone knows that.
"But we have – how can I say? - time to improve .We have a squad with potential, a new manager, so why don't we believe we can do better?
"You have to believe, you have to be positive to try to change things and to play how Manchester United fans want and the history of the club. This is why we're not going to give up. The only guys that give up are those who are mentally weak.
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“I don't want to be here at the club, to fight for sixth or seventh place, or fifth, I'm here to try row win, to compete. I think we compete, but we are not yet at our best level. But we have a long way to improve and I believe if we can change our mind, we can achieve big things."
Ronaldo, speaking to Premier League Productions and Sky Sports, added: “To win the league, it's a marathon, not a 100 metre sprint. You have to be consistent, this is why the best team, right now, in England, is Manchester City, Liverpool and Chelsea.
"They are more consistent than the other teams, but we have to look at that and believe we are capable of doing that in the same way. We know it's tough, we have to improve, some things have to change.
"But I believe that we have the potential here, so we have to do the right things. It's the only way to be at the level of those teams that I mentioned. I believe that 100 percent. I still believe. But some things have to change."