United started brightly in the new black adidas kit as Scott McTominay twice shot at goal, but the home side took a shock lead when Luis Silva met a through-ball from Jefferson Savarino to prod past Joel Pereira, prompting an explosion of noise and ticker tape from the jubilant local supporters.
Parity was soon restored thanks to Henrikh Mkhitaryan's left-footed shot that flew into the bottom-right corner from the edge of the penalty area.
United took the lead shortly after the half-hour through new signing Lukaku, who scored his first goal for the club after meeting a fine pass from the sharp Mkhitaryan.
Jose Mourinho made 11 changes at half-time, but one of those - Juan Mata - was forced to come off with an apparent injury shortly before the hour mark. Antonio Valencia was then shown a straight red card following a challenge on Sebastian Saucedo, much to Jose Mourinho's dismay on the touchline.
Aside from Luke Mulholland and Kyle Curinga missing chances to equalise for the hosts late on, United saw out the match in comfortable fashion and flew back to Los Angeles with another good workout in the bag.