Manchester City's Champions League dreams were dealt a major blow with the news that Sergio Aguero is almost certain to miss Wednesday's crunch tie with Roma.
The midweek clash could decide who progresses from Group E, although CSKA Moscow could usurp both teams, and City will have to cope without their 19-goal striker.
Aguero, who's hat-trick against Bayern Munich set up the final game showdown, has netted five times in Europe this season, but a knee injury sustained against Everton on Saturday looks likely to rule him out.
The Argentine striker went down clutching his knee as early as the second minute, and after lengthy treatment he eventually left the pitch in tears.
With Stevan Jovetic having missed the game through injury, and Edin Dzeko making his comeback from the bench, Aguero's absence leaves City frighteningly short of fit strikers.
Speaking after the 1-0 victory over Everton, Pellegrini told MCFC.co.uk: "It is impossible in this moment to know how long Sergio will be out. He has a problem with the ligament in his knee but tomorrow we will know better how serious it is.
"He will probably miss the game in Rome. We are not sure but it is very difficult when you have a problem with your ligament to recover in three days.
"It is a big problem because Sergio was in a very good moment and he is a decisive player for our team."
Following the summer departure of Alvaro Negredo, Dzeko and Jovetic are the only senior strikers at Pellegrini's disposal should Aguero's injury keep him out.
However, both have injury concerns of their own, meaning Jose Angel Pozo could be handed a first City start in the club's biggest game of the season so far.
Pellegrini said: "We must also add Edin Dzeko is only just coming back from injury, David Silva is not ready and Stevan Jovetic also has a problem.
"With Stevan, we'll see tomorrow. We hope it's just a small problem in his hamstring and we will see what will happen with him."
One man team
While the injury to Aguero will undoubtedly reduce City's chances of progression, it does give the rest of the squad the opportunity to prove they're not reliant on their top goalscorer.
Pellegrini has already insisted his is not a one man team, and played homage to his depleted squad for overcoming Everton without a number of key men.
"I am so pleased to beat Everton - a very good team - without four or five very important players. I always say this team is not just about Aguero.
"We have also missed Jovetic and Silva, and Dzeko was only able to play 20-25 minutes. All of them are very important offensive players so I think the way we won today is important for the trust in the way we defend," he declared.