The City boss then insisted his job was not on the line as he looked ahead to the fixture that will determine his side’s immediate European fate without his star striker.
Aguero, who has scored 19 goals this season and struck a dramatic hat-trick to keep City alive in Europe against Bayern Munich a fortnight ago, damaged knee ligaments in Saturday’s win over Everton.
His absence will be keenly felt but Pellegrini was keen to take the focus away from the 26-year-old ahead of the decisive Group E contest at the Stadio Olimpico.
Pellegrini, speaking at his pre-match press conference, said: “Well, Sergio – we know he is injured. He cannot play tomorrow.
“He will see how long he will be out but I think it is more important for us to think about the players we have available for an important game against Roma.”
When pressed on how long Aguero could be absent after he was assessed over the weekend, the Chilean said: “We know but it is difficult to know exactly how long he will be injured.
“It could maybe be four, five or six weeks. It depends a lot on the way he recovers.”
That would certainly rule him out of remaining games this month against Leicester, Crystal Palace, West Brom and Burnley, the New Year’s Day clash with Sunderland and an FA Cup third-round tie against Sheffield Wednesday.
Should he be ruled out for six weeks, Aguero would also miss tough games against Everton and Arsenal but he could be back to face title Barclays Premier League rivals Chelsea on January 31.
His side are level on five points with Roma and CSKA Moscow in a tight battle for the second last-16 qualification spot behind Bayern Munich.
To go through City need to win and hope CSKA do not beat Bayern, or get a score draw and hope CSKA lose. A defeat would eliminate them from Europe altogether.
Defender John Terry and forwards Eden Hazard and Willian have been rested for Chelsea’s Group G clash with Sporting Lisbon tonight.
Jose Mourinho will start with striker Diego Costa up front.