Third-placed United have won five successive Premier League games and eight of their last 10 to move above City in the table and victory in the 169th Manchester derby would extend the advantage over their neighbours to four points.
"On Sunday we have to wait and see if we can continue our good shape against City," Van Gaal told reporters. "I had more fear for Villa (who United beat 3-1 last week) than City because every player shall be motivated and want to win.
"Villa were more defensive but I don't expect City shall play as defensively as Villa did, and then you need a lot of focus and concentration on our defence.
"A long ball is very difficult to defend. But against City I don't expect very much long balls -- and that is why I have less fear against City, in spite of the loss (1-0 at the Etihad Stadium in November) in the last game."
Chris Smalling was dismissed in that defeat and Van Gaal will stress to his side the importance of controlling their emotions in what promises to be pressurised and highly-charged affair.
"I do that always, that is not different," the Dutchman added. "I have started our preparation with (discussing) the red card, (though not just) the red card of Smalling.
"I am always warning in these kind of games that we have to control our emotion. We have five red cards, unbelievable. It is a record in my career as a manager. That is not good.
"Control of emotions is a talent of a player. You have a lot of talents to have to be a professional player and you know when you have already a yellow card you don't have to make a second foul to get a second. You have to know that. But also that goal is a learning process."
City have won four successive Manchester derbies but have dropped to fourth in the table -- nine points behind leaders Chelsea who have a game in hand -- having won only four of their last 11 Premier League games.
(Reporting by Tom Hayward, editing by Ed Osmond)
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