DENVER -- Louis van Gaal hailed the footballing intelligence of Wayne Rooney after the Manchester United forward scored two goals and made another for Juan Mata in a 3-2 win against Roma.

After speaking on Friday about his philosophy of training his players "not in the legs, but in the brain," the Dutchman pinpointed Rooney's 40-yard ball to set up Mata's lobbed finish, which put United 2-0 ahead.

"The pass of Rooney to Mata -- it was unbelievable, that pass," Van Gaal said. "He did it with his brain and Mata was running with his brain at the right moment."

Rooney was at the heart of a first-half performance that saw United overcome some shaky defensive moments to race into a 3-0 lead by the interval. The England striker opened the scoring with a magnificent curling strike from 30 yards and, following Mata's goal, converted a penalty after Danny Welbeck was fouled by Urby Emanuelson.

Rooney took his pre-season tally to four goals in two games with a double strike vs. Roma. Justin Edmonds/Getty Images

However, although United were victorious in the opening game of their International Champions Cup campaign, Van Gaal was disappointed that Roma were able to fight back, which caused United to hang on to a one-goal advantage in the closing minutes.

"In the second half, we have got to keep the ball in possession because when you are 3-0 ahead, the only thing you have to do is to keep the ball in possession," said Van Gaal. "The opponent has to run and run and run and the gaps shall come."

That last season's Serie A runners-up were able to close the gap was also due to the conditions in which both sides were made to play -- the game was stopped in both halves for a water break.

Having complained before the game about the local kickoff time of 2 p.m., Van Gaal returned to the theme following its conclusion.

"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 think the system works and the players adapt but, because of [the conditions], maybe these are excuses but I don't believe when you see Mata play or [Ander] Herrera, that these are the best passers -- and also [Shinji] Kagawa -- they are the best passers and everything went wrong. It is in the air, I believe."

Van Gaal said he didn't think the crowd of 54,117 in attendance saw the best of his side and added that, had the game started later in the day, that United may have performed better.

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Meanwhile, defender Chris Smalling echoed his manager's assessment of the overall performance.

"I think at times we were a bit sloppy and we need to work on that and tidy that up," said the defender, who was substituted along with eight others at halftime.

For the second straight game, Van Gaal deployed a three-man defence and Smalling spoke about the challenges brought about by the unfamiliar alignment.

"It's a lot to learn but we're enjoying it. it's a lot of responsibility to go out and attack as a centre-half," Smalling said. "[Van Gaal] wants us to play nice and aggressive and I like to think that's my game. Communication is massive because you have to shift across and work into the space."

United travel to Washington, D.C., next for a meeting on Tuesday against Inter Milan and Smalling is keen to continue to impress his new manager.

"We're all trying to impress him, whether it's in training or around the hotel," Smalling said. "He's a man who invites a lot of respect for what he's done and what's he's like."