And Aguero's fellow Argentinian Pablo Zabaleta limped out of City's 3-1 win over Leicester in the quarter-finals of the Capital One Cup on Tuesday after suffering a hamstring injury.
With Micah Richards also sidelined, Pellegrini has a decision to make about who to field at right-back for Saturday's Barclays Premier League trip to Fulham.
Dedryck Boyata took Zabaleta's place on Wednesday, but there has also been a suggestion that James Milner could drop back into defence.
The midfielder told the Manchester Evening News: "I hope that's not the case but, if I'm asked to do it, I'll do that.
"Dedryck has come in, he's a great young player. He's got everything, he can defend well, he's quick, he's good on the ball.
"It's down to the manager. If he gets the nod, I've got every confidence in him. I think he's a really good young player and he's got a massive future ahead of him."
It has been a good 10 days for Milner, who scored the winner in City's shock Champions League victory over Bayern Munich, won a penalty against Arsenal and played a key role in two of the goals against Leicester.
The England midfielder has been in and out of the City side this season and is happy to be playing his part.
"I think I've had spells in the team before and played well as well," he said.
"It's what I do. Part of my game. Contributing to the team, creating goals - it's pleasing to be able to do that.
"For me, it's the team that comes first, either creating a goal or being part of the goal that's the main thing. I want to be part of the team and contribute as much as I can.
"The amount of games we've played this year and the amount of games we'll hopefully continue playing towards the end of the season still being in every trophy, it's important you need everyone to perform when you get that chance."
Another man taking his opportunity against Leicester was striker Edin Dzeko, who scored twice and will hope he has earned his chance in the Premier League in Aguero's absence.
Summer signing Alvaro Negredo has been Pellegrini's partner of choice for Aguero, but Dzeko told several national newspapers: '''I am a striker that has scored goals all my life, and I love to do it.
"When I get a chance I try to do my best. We have a superb squad and recently have scored a lot of goals.''
- Sports & Recreation
- Sergio Aguero
- Manuel Pellegrini
- Pablo Zabaleta
- Barclays Premier League
- James Milner
- Micah Richards