Spain internationals David Silva and Alvaro Negredo will miss Manchester City's FA Cup fourth-round tie against Watford on Saturday.
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Negredo, who is City’s 23-goal top scorer, is a doubt for the Premier League trip to Tottenham on Wednesday with a shoulder injury.
He was hurt in the final few minutes of the 3-0 win over West Ham on Tuesday when he landed awkwardly after a challenge from James Tomkins, but Pellegrini does not believe he will not be a long-term absentee.
He said: "We will see in the next days how his shoulder reacts from the work he is doing. Maybe in one week he will be OK. The first opinion [is that], yes, it is a small injury. He is not ready for Watford."
City are already without one of their attacking midfielders, in the injured Samir Nasri, and Pellegrini confirmed that Silva will also sit out the clash with the Hornets at the Etihad Stadium.
He added: "David Silva has some problems. He is not 100 percent fit. It is just a precaution -- we will not take any risks. He has some pain in his hamstring, but nothing important."
Pellegrini said he would rotate his squad for the cup tie -- as he did for the Capital One Cup semifinal second leg at Upton Park -- adding: "We are going to make some changes."
The Chilean is expecting a tough game against Watford, who were playoff finalists last season but are only 15th in the second tier this time around and have not won away for almost four months.
He added: "There is more motivation for a Championship team to beat a Premier League team, so I am absolutely sure we are going to have a very difficult game."
And he played down suggestions that City would make signings in the January transfer window, flippantly referring to rumours linking them with other players.
He said: "If we talk about the all the offers this club [is supposed to have] made, we have at least eight goalkeepers, five centre-backs, nine forwards. There is no news for the moment."