After Liverpool F.C. gave up a 3-0 lead with 11 minutes remaining to draw 3-3 with Crystal Palace on Monday, City are in prime position to claim one of the tightest title campaigns in years.
Liverpool have 81 points with one match to play while City have 80 with two matches left - and a nine-goal better goal difference - meaning victory against Villa and West Ham United on the final day of the season will guarantee City the championship While four points from the two games would almost certainly still be good enough.
City will be without their Argentine hit-man for the first of those matches on Wednesday, with the 25-year-old having been forced off midway through the first half of Saturday's 3-2 win over Everton.
"Sergio Aguero couldn't recover 100 percent from his groin problem so he is not in this squad list," manager Pellegrini told a news conference on Tuesday.
"Maybe he has a chance to be fit for Sunday. We will see in the week. I was not 100 percent sure if he was fit for tomorrow - that's why he is not in the squad."
Aguero, who will look to take a leading role in Argentina's World Cup campaign in Brazil next month, has scored 17 goals in 22 league appearances this season, but has missed many games with calf and hamstring injuries.
Midfielder Yaya Toure would be available, with Pellegrini saying his withdrawal against Everton with 25 minutes remaining was simply a precaution as the Ivorian worked his way back to fitness after hurting a muscle last month.
Pellegrini did not watch Liverpool's draw with Palace, but said it should act as a warning against any complacency within his squad.
"We have to win the game against Aston Villa, then at the end of the week, we will see which team has more points," he said.
"We will prepare exactly the same as we have done in all the games during the year. The next game is always the most important.
"It is a difficult game because all the teams are under pressure. A lot of people said Everton were not going to win because they didn't want Liverpool to win the title but we had a very tough game against them.
"Crystal Palace did not have anything to fight for but they were very strong. I am sure Aston Villa will be a difficult team for us but I hope that if we play at home as we have done so far, we can win."
Pellegrini would not be drawn on reports that City face a reported fine of 60 million euros ($83.27 million)and limits on their Champions League squads for next season for breaching UEFA's Financial Fair Play rules.
"I don't know about that," he said, repeating the straight-batted approach he has taken to the subject over recent weeks.
"Not official...nothing more to say. When there is official news from UEFA we can analyze what will happen."