The Red Devils head into the game on the back of their disappointing Europa League exit at the hands of bitter rivals Liverpool at Old Trafford on Thursday night.
United know that a win over City would move them to within a point of Manuel Pellegrini’s men heading into the final few games of the season.
And the Manchester United boss is under no illusions about the importance of Sunday’s crunch clash at Etihad Stadium.
Speaking at a news conference following United’s European exit on Thursday night, Van Gaal said: “I think so [this game is do or die]. Yeah, we are now four points behind. So you have to win otherwise the gap is bigger and bigger and the matches you have to play are fewer.
“Then it is seven points and we have to play eight matches. It’s very hard to recover from that gap.
“It’s difficult then because West Ham United are also in front. So it’s not only City but West Ham United.”
United have won just one of their last five league visits to the Etihad Stadium.
The Red Devils have taken just one point from a possible nine in their last three away games, losing to both Sunderland and West Brom.
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