United won 3-0 at the KPS as Rasmus Hojlund, Alejandro Garnacho and Bruno Fernandes struck.
Van Nistelrooy said afterwards: "I think in both boxes the difference in quality showed. The efficiency in finishing chances, and also defending key moments in the game. Overall if you look at the metrics of the game there's not much in it. But obviously the efficiency is there.
"We kept pushing and had opportunities to get back into it. Also at 2-0 we had the chance cleared off the line. We had other chances before that. It's difficult when the ball doesn't go in.
"That's what happens, that's football. You're in a moment where things don't go for you but the players are giving everything they have.
"We are not on the outside, we are on the inside. The gap is nine points, it's big, but we can't and won't give up. We'll keep going and use all the possibilities we have to turn this situation around. Now we'll have a long break and come back from there."