Van Gaal told BT Sport: "We did not reach our level what we have showed in the past matches, and that is disappointing because this is the most important match, the home match. We are disappointing the fans and we are disappointing ourselves and that's why we lost today.
"We have chances to make the 2-1, but chances is not enough. You have to make the chances. At the end the result is counting."
Van Gaal and his players will look to bounce back with a victory as they travel to Sunderland next Sunday.