Both clubs have secured seven points from their opening four Premier League matches, while the injured David Silva and Danny Welbeck could both return to the matchday squad
Both sides have battled for the crown over the last two seasons with City emerging triumphant in 2012 before United re-established their supremacy last term in Sir Alex Ferguson's final campaign in charge.
David Moyes has since taken the helm at Old Trafford and, aside from a defeat to league leaders Liverpool at the beginning of September, the 50-year-old has made a solid start.
The teams have posted identical records going into the contest - having both taken seven points from four games.
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"I don't need to be here to know how big this derby is," Pellegrini said. "Playing against Man United at home we must win, it's more important to win than to not lose."
City will be boosted by the return of captain Vincent Kompany, the defender set to make his first Premier League appearance since suffering a groin injury in the season-opening 4-0 win over Newcastle United.
David Silva is also set to be involved after recovering from a thigh injury. The Spaniard has missed City's last two matches against Stoke and Viktoria Plzen.
Micah Richards has also returned to training following a hamstring problem and could be named in the matchday squad for the first time this season, while Gael Clichy (thigh) is a major doubt.
United, meanwhile, will be boosted by the news that Danny Welbeck has returned to full training and could be available, though defenders Phil Jones (ankle) and Rafael (hamstring) are doubts for the match.
The visitors will, however, be able to call on in-form forward Wayne Rooney, who has either directly scored or provided the assist for 40 goals in his last 44 Premier League outings.
And the former Everton man insists he is enjoying life at Old Trafford again, despite having been strongly linked with a move to Chelsea during the transfer window.
"I feel good and am glad to be out playing," Rooney commented. "It's good to work under him [Moyes] and hopefully we'll be successful together. I feel great."