City revealed that Aguero would be sidelined for a month after he sustained a hamstring injury in the 5-1 thrashing of Tottenham last week.
It was reported this week that the Argentina striker could return ahead of schedule and face Catalan giants Barca in the first leg of the UEFA Champions League round of 16 tie on February 18.
Yet Pellegrini has played down those suggestions and is also preparing to be without midfielder Fernandinho, who missed Monday's defeat against Chelsea in the Premier League due to a muscle injury.
"Fernandinho is working with the physiotherapist. He will be at least 15 days more." said City manager Pellegrini at a press conference on Friday.
"It's difficult (for him to play against Barcelona). We will see in the next week but it is difficult.
"Aguero, the same thing. He's working with the physiotherapist. He's started running but he will not be available for Barcelona."
There is better news regarding midfielder Samir Nasri, who is due to return to training next week as he closes in on a return from a knee injury.
Pellegrini also expects Javi Garcia (knock) to be back in full training soon and although the duo will not face Sunderland next Wednesday, they could feature against Chelsea in the FA Cup three days later.
The Chilean said: “Samir Nasri is working well. I think that next week he will working with the squad as normal.
"He will not be fit for Wednesday, but maybe for Saturday. For Javi Garcia we hope the same."
