City reached the Champions League quarter-finals by eliminating Dynamo Kiev on Tuesday while United were knocked out of the Europa League two days later by bitter rivals Liverpool.
"If you have 24-48 hours more rest it's important, it's not a decisive thing for the game but you can prepare better," Pellegrini told reporters on Friday.
"That's the problem with the Europa League. You play on Thursday, it's not easy to prepare."
Fourth-placed City are 12 points behind leaders Leicester City with a game in hand but Pellegrini refuses to rule out his team's title chances.
"In football when you have a mathematical chance you cannot think you cannot do it," said the Chilean. "It's a winning mentality, win the next game and then see which team has the most points.
"A derby is always dangerous regardless of the position of both teams. United have a very good squad, it doesn't matter which way they play they're always a dangerous team."
City defenders Vincent Kompany and Nicolas Otamendi were forced off in the first half of Tuesday's game by injury.
Captain Kompany will be sidelined for around a month with a calf problem but Otamendi could be fit to return against sixth-placed United.
"Nico is running but it's a risk if he takes another kick on the same leg," said Pellegrini. "We'll assess him on Saturday."
(Writing by Tony Jimenez,; Editing by Pritha Sarkar)
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