City host Villa at the Etihad Stadium knowing that a win will see them move two points clear of Liverpool, who dropped points in a 3-3 draw at Crystal Palace on Monday, ahead of the final weekend of the season in the race for the Premier League title.
However, Pellegrini's men will have to cope without Aguero, with the Chilean revealing that the Argentina international will not feature due to the groin problem he sustained in the 3-2 win at Everton on Saturday.
"Sergio Aguero couldn't recover 100 percent with his groin, so he's not in the squad list," Pellegrini said.
"We'll see how he improves during the week. I didn't want to risk him, we'll see if he can play on Sunday (against West Ham)."
Despite Liverpool's slip-up, Pellegrini was quick to play down talk of City regaining their title and is wary of the threat posed by a Villa side that won the reverse fixture 3-2 in September and secured Premier League survival with a 3-1 victory over Hull City last weekend.
"It is (Liverpool's draw) just one result and it is very difficult to win games in the Premier League," Pellegrini added. "You cannot be sure you are winning until the end.
"I only know we have to win the game against Aston Villa. At the end of the week we will see who has more points.
"We prepare exactly the same as all the games during the year, we know tomorrow (Wednesday) is an important game.
"All teams are under pressure, if we play at home as we have so far I hope we win the game.
"I'm sure Aston Villa will be a very difficult team for us.
"Here at home the fans enjoy the whole season, of course if we can win Premier League it will be very important for them.
"It's not easy to do two (titles) in three years, I hope we can do it because they deserve it."
