Influential Singha captain Winothai wrong-footed goalkeeper Ben Amos to score the decisive goal early in the second half.
United had their chances, with Adnan Januzaj blazing over in the first half before both Danny Welbeck and new signing Wilfred Zaha squandered good chances, the latter cannoning a shot against the post.
But the Thai side were value for the victory and could have had more themselves as United struggled to find any rhythm at the start of their Asia tour.
Despite the visitors enjoying overwhelming support even from the home fans, it was the hosts that took the initiative and should have gone ahead within three minutes.
Skipper Winothai latched onto a simple ball over the top, beating a statuesque United back four and only a wonderful challenge from the backtracking Jonny Evans denied the striker as he dallied slightly too long over his shot.
United continued to struggle to find a rhythm but many of their best moments came from the lively Januzaj and the Belgian youngster should have broken the deadlock instead of blazing over from Fabio’s pinpoint cross with just keeper Narit Taweekul to beat.
Taweekul then needed to be alert to deny Welbeck, clawing the ball from his feet as the United striker tried to go round him.
But Singha continued to play with confidence and United had Amos to thank for keeping the scores level, a wonderful save denying Winothai as the skipper again beat the offside trap before unleashing a fearsome shot on goal.
As with the first half, it was the Singha All Stars that came out of the traps the stronger, another defence splitting pass catching Dagno Siaka’s heels when through on goal. And two minutes later, the home side were ahead, Siaka this time turning provider for Winothai to sidefoot past Amos.
It was little more than they deserved but they then had to soak up pressure in defence, Welbeck making the keeper work from a tight angle before Zaha headed wastefully wide with 15 minutes remaining.
Still United had their chances, Welbeck dragging a glorious chance wide with just the keeper to beat before Zaha curled a shot back off Taweekul’s far post.
Moyes and his team have a week to recover and come to terms with defeat whilst getting used to the stifling Asian conditions before they face the A-League All Stars on July 20.