The Argentina international has not featured for City since their 6-3 win over Arsenal on December 14 due to a calf injury.
In the striker's absence, Pellegrini's side have won five of their last six games - only failing to beat Blackburn Rovers in Saturday's FA Cup third-round tie.
Aguero is unavailable for Wednesday's League Cup semi-final first-leg clash with West Ham, but Pellegrini is keen to see the former Atletico Madrid man return ahead of Premier League meetings with Tottenham and Chelsea to come at the end of the month.
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He said: "I think Sergio will be back during January. He's working, he is training.
"When all the players recover from their injuries it's good news. Especially an important player like Kun Aguero."
Stevan Jovetic, Jack Rodwell and Jesus Navas will also miss the visit of West Ham through injury, while Micah Richards will not feature, despite recovering from a hamstring problem.
Pellegrini explained: "Richards can play but he is not in the squad because he must work a little more but he is not injured."
The Chilean also endorsed James Milner, after Theo Walcott was ruled out of the World Cup on Monday with a cruciate ligament injury.
"I can't tell Roy Hodgson which is the best choice [to replace Walcott]," he added.
"I think that Milner is a very useful player. He is always playing here and for me, I trust a lot in him. For me, he is a very good player."