Substitute striker Chido Obi scored a second-half brace for the visitors before defender Ayden Heaven headed home a cross in stoppage time for the final score.
The home side had led with a goal from Juninho 19 minutes into the match, the SAR’s first and only shot in the first half.
Goalkeeper Tse Ka-wing was in fine form for the home team, managing a number of saves in the first half. One of them was against United captain Bruno Fernandes, who couldn’t convert his back-heel flick with only Tse to beat.
Both teams made wholesale changes in the second half, with United stars Amad Diallo and Mason Mount appearing as substitutes.
But the player of the match was 17-year-old Obi, who managed a crisp, left-footed equaliser just minutes after coming on.
He then headed home a cross from Mount for the lead.
The home side had a chance to equalise on the 90th minute, but Stefan Pereira’s flick from close range was saved by Altay Bayindir.
More than 33,000 spectators were on hand, among them Financial Secretary Paul Chan and sports minister Rosanna Law, who presented the visitors with a trophy.
The victory saved Red Devils manager Ruben Amorim from a second defeat on their Asian tour, having suffered a 1-0 loss in Kuala Lumpur against the Asean All-Stars on Wednesday.