Gabriel Martinelli completed a comeback win for Arsenal in Los Angeles but of more concern to Manchester United will be possible injuries to Rasmus Hojlund and Leny Yoro.
In a frantic in Los Angeles, United forced Karl Hein into two early saves before Hojlund held off a weak challenge from Ayden Heaven to run through and break the deadlock.
Hojlund perhaps should have scored a second just minutes later, but under pressure from Ben White missed the target from close range and in the process appeared to tweak his groin. He was assessed on the pitch and soon taken off, heading straight down the tunnel.
United did not let that blow affect them, though, and remained on top as first Amad Diallo and then Marcus Rashford try their luck from close range, before the Gunners drew level against the run of play.
Fantastic work from Ethan Nwaneri on the left spotted Jesus in space into the area to simply side-foot home from six yards. Questions were immediately asked about the United defence, but replays showed that Jesus was offside but no flag was raised.
Arsenal gained some momentum from their equaliser, and again minutes later when Yoro became the second United player to be forced off with a possible injury inside 35 minutes.
Ten Hag rang the changes at the break, with the likes of Antony, Jadon Sancho and Christian Eriksen introduced, and United took a while to get back into their stride as Andre Onana was forced into a fine save to deny Leandro Trossard.
United grew back into the game as Arsenal introduced the likes of Kai Havertz, Martinelli and Thomas Party, and the German went close to a winner when stretching to tuck home a free kick delivery only to be denied yards from goal.
But it was fellow substitute Martinelli who did settle this Stateside derby, cutting inside to easily get away from James Scanlon before firing low inside the near post. The Brazilian was a threat throughout his cameo appearance, running United ragged, and deserved to clinch the winner.
Despite the Gunners winning in normal time, there was still a penalty shootout at the end of the game which United won after Havertz and Jakub Kiwior missed from 12 yards.
