If Manchester United are to finish in the Top Four then they have to beat Manchester City in today's derby, manager van Gaal said.
The Red Devils crashed out of the Europa League in mid-week meaning that their only path to next season's Champions League is through a Top Four finish in the English Premier League.
And the Dutchman says today's game is a must win one if the objective is to be achieved.
"[Is the derby do or die?] I think so, yes," van Gaal admitted.
"We are now four points behind. We have to win, otherwise the gap gets bigger and bigger. The matches that you have to play are not so big. [If we lose], then it is seven points and have to play maybe eight matches which is very sharp to recover from. It would still be possible, but very difficult I think because West Ham United is also in front as well as City.
"I never feel that I am under pressure; I do what I have to do. Do you feel like you are under pressure? People put this question to me every week and it is not so nice.
"The players are already recovering, taking ice baths and so on. We cannot train in between.
"I don't train [the players] already for two or three weeks. I train the substitutes and, because of this, Antonio Valencia can play 45 minutes tonight and that was the highest [number of minutes he could play]. I could not make the change in the first half [against Liverpool] because extra time was possible. I am always solving this kind of problem.
"What is my approach? Normally we play in the same style and try to press in a certain way. However, that is dependent on the opponents' qualities, as you saw against Liverpool.
"We have to disorganise City's defence. We know, for example, that [Nicolas] Otamendi and [Vincent] Kompany maybe cannot play so you would expect them usually to play with [Martin] Demichelis and [Eliaquim] Mangala, so we have to analyse that, and analyse City.
"I know how they play, of course, but we have to analyse them still. We shall, from this point, analyse City. I will sit down with my staff and discuss the game plan.
"It is dependable also on which players are fit to play. When you play the Liverpool match, so close to City, you have to wait and see which players are available and then that will impact on my game plan because some players may make me alter my style. It is not so easy to be a manager."