Seventh-placed United head into the game 12 points behind their neighbours, having played two games more, following an inconsistent season under David Moyes.
However, after seeing United reach the quarter-finals of the UEFA Champions League and defeat West Ham 2-0 last weekend, Pellegrini remains wary of the threat they pose.
The Manchester derby is the first of City's three games in hand on Premier League leaders Chelsea, who they trail by six points, and Pellegrini knows his side must stay focused despite United's difficult campaign to date.
"United have had a difficult season, but they're a very good team," said the 60-year-old. "We're sure we must have a very good game if we want to win.
"The players are very motivated to try and win the title; it was a very close Premier League from the beginning of the season.
"We are now just focusing on the Premier League. Physically and mentally we are focused on trying to win the title.
"Our target is to add three points, but we don't just need to win tomorrow, but (also) the next games until the end of the season."
City will once again be without striker Sergio Aguero due to a hamstring injury, while Matija Nastasic is not ready to return following a knee problem.
However, captain Vincent Kompany is set to play at Old Trafford after serving a one-match suspension.
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