Sergio Aguero, Mario Balotelli and Edin Dzeko have scored 43 times between them in all competitions this season but their lack of goals in recent weeks has compounded the team's loss of form. City endured a difficult January, losing twice in the Barclays Premier League and being knocked out of two cup competitions.
Top scorer Aguero has not scored in five, Balotelli is serving a four-game suspension and although Dzeko has scored two in his last four matches, he has spurned a number of chances. Mancini, thinking back to City's free-scoring start to the campaign, said: "People were saying how many goals City could score - 100, 200 - but this is impossible."
He added: "There are some moments when you score three or four goals but others when you can't score any goals. It is impossible to score three or four goals every game for 38 games. Probably when Sergio, Mario, Dzeko start to score goals - that moment will probably change our situation."
Questions have been asked of City's ability to handle the pressure of the title race after losing 1-0 at Everton on Tuesday. With champions United now trailing only on goal difference, City face a test of their nerve as they host Fulham at the Etihad Stadium on Saturday evening.
Mancini said: "I think until now we have had a fantastic season but we know that if we want to win the title, you can't win it now. You should fight until the last two or three games because United have big experience."
Mancini has no doubt about his ability to maintain the fight, having previously guided Inter Milan to three Serie A titles. He said: "There doesn't exist a place like Inter Milan, where you can really be under pressure. After Inter Milan, you can manage every team.
"Inter Milan has a big history like United. When you are a manager there you have a pressure every day, every minute. After Inter Milan you can't have pressure."
City could hand a debut to new loan signing David Pizarro. The 32-year-old former Chile international joined the club until the end of the season from Roma just before Tuesday's transfer deadline.
City are also without Kolo and Yaya Toure, who are still at the African Nations Cup with Ivory Coast.