The Reds held out but van Gaal cut a disgruntled figure afterwards, bemoaning the heat, altitude and 14:00 kick-off time.
"I am pleased that we have won but it was not a good performance," the Dutchman said. "54,000 people came to watch us but we can perform much better.
"We are not used to this hot weather. I think we didn't play a good match because of the height of the stadium, the air, because all my good passers failed today.
"I said when the World Cup was here in '94 it wasn't wise to play here at noon. America has a top sports climate but you have to play football at the right time.
"When you are 3-0 ahead, the only thing you have to do is keep the ball in possession and then the opponent has to run and run and run. It's very difficult in this hot weather to do that.
"But we forgot to do that [keep possession] and we brought Roma into the game ourselves."
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