Manchester City boss Manuel Pellegrini has warned his defenders they must raise their levels. Pellegrini said: “I think that we must improve. It’s very important to have a clean sheet because if you are always conceding one goal before you score you are playing against time, against the rival, the score, against a lot of things. “One of the things that we did last year was improving clean sheets. “We had a second round (of fixtures) with a lot of clean sheets. “At this moment last year we had conceded more goals, but in the last three games in Premier League we have conceded five goals and that is too much. We drew two games and won one, but it’s too much.” Pellegrini admits he is still unsure what his first-choice central defensive pairing should be. Martin Demichelis was restored alongside influential captain Vincent Kompany for the midweek Champions League draw against Roma after £32million summer signing Eliaquim Mangala had a poor game at Hull last Saturday. Demichelis and Kompany had started the season together while Mangala built up his match fitness. Pellegrini said: “I cannot say because it depends on their performance. Press Association “We bought Mangala for an important amount of money because we thought that we needed a centre-back, but I’m very happy with the performance of Martin and Vincent Kompany, also. “Mangala was not out because he made a mistake against Hull on Saturday. “We are just starting the season and we must finish in May so we must have a lot of games to play. “Of course players not in a good performance cannot play, but it’s important to make a good rotation, it’s important to decide tactical things. “For that game I decided that it was better to play Demichelis, who was also playing very well. “It was not linked with what happened against Hull and I think he (Mangala) will be a very good defender for us during the whole year.” Pellegrini also has strong competition for places in midfield. Yaya Toure and Fernandinho were the first-choice pairing last season but this term Pellegrini has the extra options of Fernando and Frank Lampard. Fernando impressed in the opening weeks of the season before suffering injury while former England international Lampard, signed on a six-month deal from sister club New York City, has shone of late. Pellegrini said: “I have four very good midfielders because we have to play around 60 games during the whole year. “We’ll see which midfielders are in the best moment to play each game. That’s never a problem when you have good players, it’s a problem when you don’t have good players.” Pellegrini will be without the bulk of his squad next week as they head out on international duty. Fitness concerns over Liverpool’s England striker Daniel Sturridge have reignited the club-versus-country debate this week but Pellegrini has no complaints about the situation. The Chilean said: “Of course you have concerns about that but you can do nothing. “If I am in charge or managing a national squad, I (would) hope that all the managers will help me. I think it’s our duty to help all the other managers. “If not, you cannot have World Cup, you cannot have European Championships. It’s an obligation for us to do it.” When City’s players return to club training, they will do so not at their familiar Carrington base, but at the new £200million City Football Academy. The state-of-the-art complex adjacent to the Etihad Stadium will start opening in stages from next week. Pellegrini said: “It is unbelievable. I have been in another important training ground at Real Madrid, but this one I think is the best I have seen.” The Barclays Premier League champions have kept just one clean sheet in their last eight games, and that came in a 7-0 thrashing of Championship side Sheffield Wednesday in the Capital One Cup. City have not shut out the opposition in the Premier League since they won 2-0 at Newcastle in their opening match of the season.