However, that result means West Brom are now without a win in 18 Premier League games and moored in the bottom three.
It was another frustrating day for Mourinho, whose side had crashed to an embarrassing League Cup quarter-final defeat at second-tier Bristol City in midweek. However, they were still second in the table, and had accumulated the most points at this stage of the season compared to any United side in the post-Ferguson era.
The Red Devils could only manage a 2-2 draw with Leicester last week.
"Manchester City buy full-backs for the price of the strikers, so when you speak about big football clubs, you are speaking about the history of the club".
In the north London club's first away win since September, Harry Kane's hat trick secured a 3-0 win at Burnley and a rise into fifth place.
Stoke fought back and had chances through Xherdan Shaqiri and Peter Crouch without finding a breakthrough, while home goalkeeper Jonas Lossl also played his part, producing fine saves to deny Ryan Shawcross and Eric Choupo-Moting.
These kind of results make people sit up and take notice.
"Nothing at all negative to say about my players".
United looked risky after the break, the result of which The Red Devils bore in the 61st minute when Mata once more found the net and this time with a direct free kick.
Ashley Barnes' opening goal was significant for another reason.
Mourinho's side have now shipped in five with this latest defeat handing the Reds a chance to close the gap to them to five points.
The visitors had a number of chances to put the game to bed, but a lack of killer instinct - an all common theme last season - cost them maximum points.
United's players looked ashen-faced as they traipsed onto the bus at Leicester, where they were on course for a comeback win after Mata's first top-flight brace since March 2015 cancelled out Jamie Vardy's opener.
The England worldwide was more dynamic than Zlatan Ibrahimovic and proved more adept at getting in the right place when it mattered. The big mercurial Swede is yet to score since his return to first-team football in late November, and today - after making his first start for United since the injury - he was hooked off at half-time.
"There are many big clubs".
Mata put United 2-1 ahead with a superbly-taken free-kick and revealed afterwards that his goal celebration was a dedication to Indian children with whom his charitable foundation Common Goal works. The Argentinian's rash streak was there for all to see straight from the off, conceding the free kick with an elbow after two minutes that ultimately led to the Burnley goal.
Substitutes: Jesse Lingard (for Ibrahimovic, 45) 8, Henrik Mkhitaryan (for Rojo, 45) 6 Burnley: Nick Pope 8, Phil Bardsley 7, Kevin Long 7, Ben Mee 8, Charlie Taylor 7, Johann Berg Gudmundsson 7, Steven Defour 7, Jack Cork 6, Scott Arfield 7, Jeff Hendrick 7, Ashley Barnes 7. Some players are leaving the pitch after 15-20 minutes, some others don't resist for the 90.