Mata had opened the scoring at the Liberty Stadium but goals from Andre Ayew and Bafetimbi Gomis eventually secured victory for the Swans.
It was a familiar feeling for Louis van Gaal, who lost home and away to Swansea last season.
Mata was understandably downbeat following the match as United went home with nothing.
“It is a big disappointment. We scored first and then we lost the game in five minutes with two goals and couldn’t score again,” he said.
“We’re disappointed because we did the same thing as last season against Swansea and lost 2-1.”
United simply couldn’t finish their chances in the game and they were made to pay as a result.
Louis van Gaal sent on Marouane Fellaini late on once again as he searched for an equaliser and lumping the ball up to him really is an embarrassing tactic for a club of United’s stature. Ed Woodward and van Gaal now face a crucial few final days in the transfer window which could make or break the upcoming campaign.