For United, the prospect of Champions League football next season remains still alive but in the balance after a solid if unspectacular win at relegation-threatened Norwich.
Once again, the Reds were dealt a rotten hand by injuries.
Anthony Martial picked up a knock in the warm up and then Matteo Darmian was stretchered off with a nasty-looking ankle injury after 12 minutes to surely put his FA Cup final participation in doubt.
Martial's injury prompted a late change from Van Gaal as Wayne Rooney was pushed up front and Ander Herrera was promoted into midfield.
The captain was a peripheral figure for much of the match- indeed the only visiting player to have fewer touches than Rooney in the first half was the aforementioned and unfortunate Darmian.
The England skipper did improve after the break and it was his tenacity and work-rate that allowed Mata to notch the decisive goal and keep our Champions League hopes alive.
United controlled the game throughout- to an extent where you could never envisage us losing- but our attacks lacked pace and zip, a complaint often made this season by critics of Van Gaal's tactics.
This match followed a familiar LVG-imprinted pattern: 67% possession, but only two shots on target and none in the opening half an hour.
At this stage of the season, though, it's all about results and United's three precious points are all that Mata.
A better side would have made us pay for the paucity- an injury ravaged and below par United side were there for the taking but Norwich were tentative, their decision-making poor and finishing worse.
They massively let us off the hook and showed why they looked doomed for an immediate return to the second tier.
It is unlikely that West Ham will be so charitable on Tuesday when we visit Upton Park in the last ever match at the 112-year-old stadium.
Spare a thought for Memphis Depay- it is just not happening for him at United.
Restored to the starting line-up with Martial's injury and Rashford rested, this was a rare opportunity for the young Dutchman to really show what he could do and perhaps nail down a regular place in the side.
He blasted countless free-kicks straight into the wall or over the bar, he couldn't find a team mate if he tried and he appears to be bereft of self-belief and suffering a crisis of confidence.
Memphis hasn't had enough opportunities of late but he had one here and- on this evidence- he might have to wait a while for his next go.