The Whites were undone 3-1 by the petroleum side, who came back from behind to register the first-ever victory over their Cairo rivals.
Hassan admitted his players were out of sorts, but indicated he could do nothing about it.
"The players were spiritless during the whole game, and I don’t know why," the Egypt icon said after the game.
"There were eight or nine players who were completely out of form, and of course I cannot take them all off at once.
"Maybe the players got complacent after the early lead, but then they failed to make good use of it.
"We are still working in the psychological factor because this team got used to defeats in recent years. Some players have a problem with confidence even when they are in front.
"The players know I am angry, but I am also confident they can bounce back in the following matches."
Zamalek, who slipped to seventh place on four points, host Gouna next month in their upcoming Egyptian league fixture.