Roberto Mancini believes Manchester United are showing signs of fatigue just when Manchester City are enjoying their strongest spell of the season as they try to catch their rivals, who are 12 points ahead in the Premier League.

In their past two matches, United have been knocked out of the Champions League by Real Madrid and then surrendered a two-goal lead to Chelsea in the FA Cup quarter-finals before holding on for a 2-2 draw and a replay. Sir Alex Ferguson and his defender Jonny Evans have admitted to mental tiredness.

Asked if United are weakening, Mancini said: "Football is strange for this. Sometimes you lose a game, you don't know why. Maybe one, two, three games. We took two points from three games against Liverpool, Southampton, QPR [in late January and early February]. Sometimes this can happen. United are playing very well this season, they played fantastic against Real Madrid. But 10 games until the end is a difficult moment because all the teams can be tired after eight months. We hope."

Mancini believes United's run of form proves they are suffering from fatigue. "For this reason – because when you play one game every three days, it is always a top-quality game, there will be a moment that you are tired. I was not surprised [about the Chelsea result]. Sometimes they didn't play very well but won because they have good players. For a top team it is important to win when you don't play well."

Are City stronger now? "Yes. I think so. [Saturday] will be an important day for us. The next two or three games will be crucial. If they lose a point in the next two games, maybe [we can catch them]. After the two games, we have the derby [on 8 April]."

Mancini also admits that the club erred when they failed to sign a central defender in the winter transfer window as cover for Vincent Kompany, who injured a calf at Stoke City on 26 January. This was the game immediately before the loss at Southampton and draws with Queens Park Rangers and Liverpool to which Mancini referred.

Kompany is still out though the Manchester City manager hopes that he, along with Sergio Agüero who is back in Argentina as he recovers from a knee injury, can return following the international break.

"Probably, we made a mistake," said Mancini. "We knew the problem and we were four days to the close of the market. Probably there, we should have bought another defender. We tried. We had a chance. But we didn't think that Vinny would stay out 45 days. This was a big problem. Also, last year he had this problem for one month he didn't play. It was a problem.

"No, I don't think he is out for the season. He has started to run. We hope and think he will be ready for Newcastle. He started with us four days ago but he was tired because he worked a lot. His calf was tired but it is no problem.

"We don't want to take a risk at the moment. We have one game then the international break, better that we wait.

"The calf is a dangerous injury. If you don't recover well, you can have a problem for four, five, six weeks in a row."

Regarding Agüero, who has not played since the 2-0 win over Chelsea on 24 March, he said: "Sergio is in Argentina but he comes back Sunday and Monday he starts to run here. He needs to do swimming pool strength. I think he will be ready for Newcastle."

Mancini is also surprised that Roy Hodgson did not select Joleon Lescott for the England squad for the upcoming World Cup qualifiers against San Marino and Montenegro. "I don't know why he's been left out. I read all the newspapers, they were pushing for Rio [Ferdinand]. He has a lot of defenders, but Joleon is a good player."

The FA has confirmed the dates and kick-off times for the FA Cup semi-finals at Wembley next month. The clash between Millwall and Wigan Athletic will take place on Saturday 13 April at 5.15pm, live on ESPN. City will take on either Chelsea or Manchester United on Sunday 14 April at 4pm and it will be shown live on ITV1.

Chelsea and United play their sixth-round replay at Stamford Bridge on Monday 1 April.