Wayne Rooney’s eighth-minute header saw United edge an evenly-matched first half at the Levi’s Stadium – home to NFL’s San Francisco 49ers – before substitute Jesse Lingard doubled the Premier League side’s lead shortly after the hour mark in California.
Rafinha’s exquisite volley on the cusp of full time appeared to have set up a grandstand finish in Santa Clara, but Adnan Januzaj punished Marc Bartra’s hesitancy when dealing with a long ball moments after to seal United’s third consecutive pre-season victory.
United will now travel to Chicago where they take on Ligue 1 champions Paris Saint-Germain in the final match of their tour of the United States.
Luis Suarez endured a frustrating afternoon in the California heat. The former Liverpool striker hit the woodwork twice and was subjected to some rough treatment by Phil Jones.
David de Gea made his first appearance of the tour after overcoming a back injury, but there was no sign of Angel Di Maria, who looks to be moving closer to a switch to PSG.
Jones assured Suarez was made to feel unwelcome early on, clattering into the forward from behind.
A minute later the Uruguayan beat De Gea with a curling free-kick, but luckily for the Real Madrid target the ball struck a post and bounced to safety.
De Gea was at full stretch moments later to deny Sergi Roberto, who had been played in by Rafinha. But, with Barcelona dominating possession, it was Van Gaal’s men who struck first.
Young floated a corner into the box and Rooney managed to outmuscles his marker Adriano before powering a header past Marc Andre ter Stegen.
United were playing a dangerous game, trying to pass out from the back while Suarez, Pedro and Rafinha lurked on the edge of the box. However, they looked more impressive going forward as the first half progressed, though.
Rooney rounded the keeper but could not finish before Ter Stegen tipped Young’s fierce drive over the bar.
Suarez ended the half on a frustrating note, hitting the woodwork once again from inside the box.
In the second half, Memphis Depay showed his strength, turning his marker and firing a low shot at Ter Stegen as United looked for a second.
Van Gaal made 11 changes just after the hour and, less than three minutes later, United doubled their lead.
Tyler Blackett played a one-two with James Wilson before crossing to the back post where Lingard drove home from 10 yards.
United looked to be coasting to victory, but Rafinha hammered a superb volley past substitute goalkeeper Sam Johnstone in the dying minutes.
But Januzaj restored United’s two-goal lead shortly after, putting the result beyond any doubt when he held off his marker before driving home from inside the box.