City have found themselves in a battle to finish second after a run of poor form—coupled with improved results from United and Arsenal—has seen them slip to fourth in the table.
They began the season focused on retaining the title they secured last May, but defeats to a number of teams they would be expected to beat have seen them lose position in the league.
All of which makes Sunday’s match with United hugely important. City have dominated this fixture in recent seasons, with United’s last derby win coming in 2012, but the improvement Louis van Gaal's side have shown in recent weeks means it is likely to be a tight game.
Kompany, who turned 29 on Friday, faces a fitness test on Saturday, having suffered a hamstring injury in the defeat to Crystal Palace on Monday night. Striker Wilfried Bony will definitely miss the game after twisting his knee and ankle in training. Stevan Jovetic and Dedryck Boyata are also out.
“Kompany: We will assess him tomorrow,” Pellegrini said. “He could finish the last game but he didn’t work this week in a normal way. We will see tomorrow if he can play or not.
“I always give the same answer. It is always better to play with Kompany but if for different reasons he cannot play, we have a squad and he must be replaced.
“Stevan Jovetic has a muscle injury. Bony had a problem the day before the game against Crystal Palace, he twisted his ankle and his knee so he will not be ready for this game. Boyata has a problem in his knee.”
The loss of Bony is a blow given he was showing some good form before his recent setback, but it’s the fitness of Kompany which will have City’s management team sweating the most.
He has struggled for form this season, with a string of poor displays raising question marks over his place in the side.
Indeed, for the win over Leicester City at the Etihad recently, he was dropped to the bench for the first time in his City career.
However, he remains an important player, and his performances in big matches over the years are proof of his ability to lead the side in the most difficult circumstances.
City have won the last four matches against United, and a fifth on Sunday would be a new club record. It would also see them leapfrog United and move into third.
However, Pellegrini said his side were focused on winning the game, regardless of setting a new record.
“I think the first motivation is always to win. After that when you have that motivation is against whom you are going to win, being a derby against a strong team like Manchester United that they dominate the league for so many years.”
The form of Sergio Aguero is a concern, with the striker having gone six matches without a goal, but he enjoys this fixture, having scored six goals in seven matches against United, including four goals in his last three.
City need him back to his best against a defence which has struggled at times this season.
But Pellegrini's main concern right now will be the fitness of his captain. In fixtures like the one City face on Sunday, leaders are vitally important, and regardless of his troubles this season, Kompany remains the club's most prominent leader.
Rob Pollard is Bleacher Report's lead Manchester City correspondent and will be following the club from a Manchester base throughout the 2014-15 season. All quotes are obtained firsthand unless otherwise noted. Follow him on Twitter: @RobPollard.