United's second goal did arrive after 20 minutes, and again it came as they won possession back inside the Galaxy half, with Ander Herrera stealing the ball before giving it to Mata.
Earlier, England striker Marcus Rashford scored twice with Marouane Fellaini, Anthony Martial and Henrikh Mkhitaryan scoring United's other goals in a one-sided victory in front of 25,667 fans. "We wanted four new players, fifty percent of the job is done".
Galaxy full-back Nathan Smith was caught in possession by Jesse Lingard, who poked the ball through for Rashford to curl his finish into the bottom corner with aplomb and get United's 2017-18 campaign off to a flawless start.
"Comparing Rooney with Lukaku is comparing the incomparable", Mourinho said, adding that the 24-year-old Belgian serves as a focal point of the team's attack.
"We made a decision to play more matches with three in the back".
He should have scored a third in the 43rd minute when he only had goalkeeper Jon Kempin to beat, but took one touch too many.
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"It's just the natural evolution of a kid", Mourinho said.
Watch the three first-half goals below.
"I thought it was a real positive experience", Onalfo said. "We have players who can't play football for the next few months".
We'll update this post with team news once the starting line-ups are announced. A real positive experience to get us ready for a real dog fight in the second half of the season.
But Lukaku could not get on the scoresheet despite two good second-half chances.
Manchester United continue their North American tour against Real Salt Lake in Utah on Monday, followed by high-profile friendlies against Manchester City, Real Madrid and Barcelona before the side heads back to Europe.
De Gea has always been linked with a move to the Spanish capital, but Mourinho has reportedly already informed the club that he does not want the shot-stopper to join Real. The 22-year-old France midfielder has been the subject of a long drawn pursuit all summer, and has now signed for the Premier League champions on a five year deal, a move that is sure to hasten the Serbian midfielder, Matic away from West London.