Tomas Soucek and a Jannick Vestergaard own goal saw West Ham to a 2-0 win.
Van Nistelrooy said afterwards: "The goals you concede are easy goals. It's a reflection of how we played in the first half. If you see the difference in the first and the second half, how we as a team moved forward to press the opponent, play with urgency after winning duels, in the first half we were waiting way too much. We have to put the opponent under pressure in the right way.
"We were way too passive (in the first-half). We are in a situation where we are not winning games and we are in a situation where we are in the bottom three. The pressure is on. The way to get out of this is to move forward literally on the pitch as well. It doesn't mean you have to go and press in a one v one situation, but we have to take initiative. We cannot wait and sit back. That is the situation we are in.
"If you play like this in the first half you are not going to score. We were not even close to scoring. In the second half we were much closer."
On how he keeps morale up, Van Nistelrooy added: "By believing in work, in turning this around, by literally moving forward, not sitting back. That is what we need to do."