“Maybe it’s not so bad as we thought at the beginning but he needs at least ten days or two weeks to recover.
Striker Sergio Aguero and defender Vincent Kompany are fit to start against Sunderland as third-placed City look to close a seven-point gap on leaders Liverpool.
Pellegrini’s team, on 70 points, have two games in hand on Liverpool and second-placed Chelsea who have 75.
Toure hobbled off with a groin injury after 20 minutes in Sunday’s 3-2 loss to Liverpool, with Chilean Pellegrini indicating afterwards he could miss the remainder of the season.
A two-week layoff would rule Toure, a driving force in the centre of City’s midfield, out of the visit of West Bromwich Albion next Monday and potentially the trip to Crystal Palace on April 27.
Kompany hurt himself in training on Saturday in a collision with team mate Micah Richards but was passed fit to face Liverpool.
The move backfired, however, as the captain and Belgian international botched clearance allowed Philippe Coutinho to score the winner with 12 minutes remaining.
Argentine Aguero came off the bench against Liverpool, his first appearance in a month after being plagued by a hamstring problem.
The loss to Liverpool means even if City win their remaining six matches, they must rely on the Merseysiders to drop points if they are to claim their second title in three seasons.
“The team knows perfectly what they have to do – we still have six more games and we will fight to the end for the title,” Pellegrini said.
“If we don’t do it, it’s because Liverpool have won their four games, I don’t think anything changed on Sunday.
“We must win our games, we can’t do anything about other teams. We know exactly what we have to do – we start tomorrow, hoping to win one of the postponed games.”
Bottom club Sunderland are running out of time to save their Premier League status, but having given City a scare in the Capital One (League) Cup final last month, Pellegrini warned against taking anything for granted.
“Playing against a team in the relegation spots is very difficult – nobody can think that one of those games will be easy,” he added.
“I am sure they will try to fight to the end to stay in the Premier League.
“It’s a different game to Wembley – one thing doesn’t have a relationship with the other.
“It’s difficult when you don’t win the game before but as I said, the players are conscious that we are fighting from now until the end of the season, so I am sure we will see something from them tomorrow.”
(Reporting by Josh Reich, editing by Ed Osmond)