Van Gaal made it two wins from two as United boss thanks to a 3-2 win over the Italian giants in the Colorado heat.
Unlike in their 7-0 thrashing of the Los Angeles Galaxy, Van Gaal's men were slow off the mark and Roma wasted numerous opportunities to go ahead.
Van Gaal was annoyed with the way his team failed to close the game out after going 3-0 up, but he thinks the fact that the game took place in 30 degree heat, some 5280 feet above sea level, did not help his players.
"I think we don't play a good match," Van Gaal said.
"All my good passers failed today. I think it was because of the height and the air.
"Mata, (Ander) Herrera and (Shinji) Kagawa these are the best passers (at the club), and everything went wrong.
"In the second half we forgot to keep the ball.
"When you are 3-0 ahead the only thing you have to do is keep the ball in possession and the opponent has to run and run and run.
"We forgot to do that. We brought Roma by ourselves back into the match so I am pleased we have won, but it was not good."
Rooney collected the ball from Mata and curled a stunning shot over Lukasz Skorupski from 25 yards to put United ahead.
The England striker then returned the favour, firing a 45-yard ball over the Roma defence to Mata, who killed the ball with his first touch before superbly lifting it over Skorupski.
A clumsy foul by Mehdi Benatia on Danny Welbeck allowed Rooney to add his second from the penalty spot.
Van Gaal was reluctant to single anyone out for praise, but he could not hide his admiration for Rooney's performance.
"The pass of Rooney to Mata was unbelievable," said Van Gaal, who withdrew the England striker, along with eight others at half-time.
Van Gaal was not so happy with Amos for letting in Miralem Pjanic's 60-yard shot.
Pjanic spotted Amos well off his line in the second half and punted a looping shot over the 24-year-old goalkeeper.
When asked whether he regarded the goal as a wonder strike by Pjanic or an error on the part of his substitute, Van Gaal said with a smile: "What do you think?
"I am nearly 63. When the ball is flying in the air for four seconds, I think I run 50 metres."
Roma coach Rudi Garcia was stunned by the 60-yard effort.
"It was a world-class goal," he said.
"I am sure we will see it throughout (the) world. He probably had a training session yesterday with the Denver Broncos."
Van Gaal, meanwhile, played down suggestions that he does not see Wilfried Zaha as part of his first-team plans.
Zaha is the only player in Van Gaal's 26-man squad not to feature in this tour of the USA.
But the former Holland boss says he has no issues with the forward, who was loaned to Cardiff last January.
"I have a lot of players in (his) position," Van Gaal added.
"Our squad is not balanced.
"Maybe Zaha will be picked because he is working good and hard, and is doing all I require and demand. There is no problem (with him)."
Van Gaal and his squad fly to Washington DC on Sunday ahead of their next match in the International Champions Cup against Inter Milan, which takes place on Tuesday.
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- Van Gaal
- Wayne Rooney
- Juan Mata