Sunderland saw the gap between the relegation zone and safety grow after this defeat at Manchester United.
The Black Cats competed strongly throughout but fell behind to a Daley Blind goal just before the break, with Zlatan Ibrahimovic and sub Henrikh Mkhitaryan adding late strikes.
Fabio Borini added a late stunner for David Moyes's side.
Burnley's 1-0 win over Boro at Turf Moor meant the gap from Sunderland in 18th to 17th is now three points.
It was to be an unhappy return to Old Trafford for Moyes but there was plenty of effort from his side.
A hug from Fred the Red, handshake from Mourinho and clutch of autograph hunters welcomed Moyes into the away dugout at Old Trafford, where the view in the opening stages was far from comfortable.
However, United were unable to make any real inroads during their enterprising start as Sunderland's threat grew in tandem with their confidence.
Papy Djilobodji saw an effort blocked and Jermain Defoe threatened on the counter, before David de Gea scampered across his goal to prevent Patrick van Aanholt scoring a free-kick.
Play was swinging from end-to-end and United broke with Juan Mata looking to latch onto a long ball, only to be barged to the deck with some force by Lamine Kone - a penalty appeal ignored by referee Martin Atkinson.
Mourinho's visible chagrin was nearly lifted by the impressive Paul Pogba, who saw a first-time strike hit the post via a slight deflection from Seb Larsson.
The world's most expensive player tried an effort from distance and hit a dipping volley as he attempted to lift a United side who had De Gea to thank for smartly denying Victor Anichebe and Borini following a defensive mix-up.
Mourinho's side were not performing at the best but managed to eke out a first-half opener. Ibrahimovic held up the ball on the left-hand side of the box and teed up Blind, who took a touch on the run and drove the ball past Jordan Pickford.
Pickford prevented the home side from extending their advantage, diving low to his right to keep out Mata's stoppage-time free-kick.
United continued in the ascendancy after half-time but Pogba powered a header just over and Ibrahimovic missed the target with a fizzing strike.
Mkhitaryan's arrival raised the quiet home support and the Armenian curled wide within moments of his introduction, then impressively slipped through Ibrahimovic to get away a shot Pickford denied well.
The Swede would get the better of the impressive Black Cats goalkeeper in the 82nd minute after possession was frittered away by Didier Ndong, with the veteran to be fed by Pogba and slotting home.
Ibrahimovic swung in a cross from the right flank and Mkhitaryan directed home with an outstanding flick in front of the Stretford End.
It was a remarkable goal, albeit from an offside position, that Borini tried to match with his own wonder strike, but it was a mere consolation.