Manuel Pellegrini's quest to lead Manchester City to an unprecedented quadruple is boosted by Sergio Agüero's return from injury for the FA Cup third-round replay against Blackburn Rovers on Wednesday, with the manager shrugging off the loss of Samir Nasri for six weeks by pointing to the strength of his squad.

City are the only club who can still win the Premier League, Champions League, FA Cup and Capital One Cup. Pellegrini pointed to his side's exceptional form in Agüero's absence because of a calf injury as a sign that they have the mentality for this challenge.

The striker had been City's standout performer with 19 goals in 20 appearances until he suffered the problem following the 6-3 win over Arsenal on 14 December. In the subsequent eight games City have won seven and drawn the other, the first tie at Roverso, scoring 22 goals and conceding only seven. The forwards Álvaro Negredo, with six goals, and Edin Dzeko, with four, have stepped up to ensure Agüero has not been missed.

With Pellegrini stating that Nasri may be kept out by a knee injury for less than the eight weeks predicted by the midfielder on Monday, the manager said: "Sergio will always be an important player for us, not only because of his goals but he gives us a lot of play during the match.

"Negredo is doing very well and we have Edin Dzeko as well who is a very important striker for us. When Sergio was injured Edin started to play often and he scores a lot of goals for us. All of the players in the squad are very important."

Of how the manager hopes Agüero will play on his return Pellegrini said: "You never know but I hope that he will come back the same way as before he was injured. He was in a good moment and maybe he needs two or three games to return to that level but Sergio will do it."

Nasri was injured when Mapou Yanga-Mbiwa hacked at him in Sunday's 2-0 win at Newcastle United. The Frenchman has been one of City's majort performers this term and Pellegrini said: "He's a very important player but I hope he can come back in five or six weeks. It's important for the rest of the squad but it's the same thing that happened when [Vincent] Kompany, Agüero or [David] Silva were injured. We always had another player that replaced him and we will try to do that with Samir but he was in a very good moment."

Nasri is not happy with Yanga-Mbiwa. "Of course he's very upset because he has to stop at this moment for six weeks with a serious injury. He cannot be happy but these things happen in football," said the manager. "It's important that you can complain because we are not ice men, so of course we have to complain about some decisions but not every decision. The referee made a mistake and nothing happened. It was a pity for Samir."

Nasri's injury will not affect City's strategy during the current window. "No, it has not changed our philosophy," Pellegrini said. "We know exactly what we are going to do in the transfer window."

The 60-year-old is conscious that to win the quadruple is not easy. "It's very difficult but we will try – it's important to have the mentality at least to try," he said. "During the season with injuries and the amount of games we have to play it's very difficult and that's why no one's done it before. But you must have the mentality to at least try to do it and that's what we're trying to do at this moment.

"We're not thinking about winning the four competitions, we're just thinking about Blackburn in the FA Cup and then to try and win against Cardiff on Saturday because until the end of the season we are going to be with five or six teams fighting until the end and it will just be decided by one or two points."

Yaya Touré and David Silva are not available because of minor injuries. Each is expected to return against Cardiff and of the Ivorian Pellegrini said: "Yaya has the last two weeks played with a problem with his back, so it is a risk to continue doing that way. It is better for him to rest for two or three days."