The red card ended the game as a contest, but did it have a real impact on the result?
Almost certainly not.
Once Seb Larsson was harshly sent off for a late challenge on Ander Herrera, the Black Cats were always vulnerable.
That United did not score more said more about their wastefulness on the counter, an inability to turn numerous promising positions into goals. It made it even harder for Sunderland to keep the ball, a task they have struggled with even with 11.
But by the time Larsson left Sunderland were already behind, Zlatan scoring a fine individual effort.
Sunderland's spell without a goal passed the 10 hour mark and even with a stand in goalkeeper for the opposition, they did not look particularly like bringing this most painful drought to an end.
Henrikh Mkhitaryan ended it just after half-time, given too much space to shoot and score. The third came as the Stadium emptied, Rashford adding the gloss to a another pedestrian defeat for Sunderland.
It was a game played in the first half with a strangely pedestrian tempo, belying the need for united to press for the top four and Sunderland to offer some hope that they could launch an unlikely escape.
The opening exchanges were all United, Luke Shaw, controlled by Jose Mourinho or otherwise, getting in dangerous positions down the left and regularly creating an overlap. He teed up Jesse Lingard just a minute in, the winger forcing an early but comfortable save from Jordan Pickford.
That said possession was not turning into chances for the most part, Lamine Kone and Jason Denayer not looking greatly comfortable but not scrambling either.
The Black Cats were clearly relieved to have Victor Anichebe back, taking the long balls and turning them into something promising. Just 12 minutes in he forced a corner, backing into his opponent and rolling him with ease.
Bryan Oviedo forced a save from stand in goalkeeper Sergio Romero, but it it was a weak free-kick that never threatened to find the net.
The hosts forced their best opening a quarter of the way into the game, but frustratingly were behind just minutes later. Good build-up saw Anichebe lay-off to Ndong, who in turn found Cattermole. His drive from distance was firm but straight at Romero, whose superb clearance found Lingard, who drove at goal and forced a save from Pickford.
Sunderland looked to avoided the danger as the corner was cleared, but Zlatan was allowed to turn by Jones and Kone, firing into the far corner.
An effort impressive and eminently avoidable from a defensive perspective in equal measure.
The game was effectively settled just before the break, Seb Larsson sent off to the bemusement of the Stadium of Light. He had pinched the ball from Pogba and just arrived late as Herrera gathered the follow up.
He made contact certainly, and one foot was off the floor, but it was neither particularly wild or forceful.
Victor Anichebe had gone close moments before, rolling his defender again and nearly beating Romero, but the game now looked done.
The second came just moments after the break, Sunderland far too slow to sense the danger down their left flank. Shaw again advanced, finding Mkhitaryan, who spun away from Cattermole. He ran towards goal, and found no resistance from Lamine Kone, too far off, backing away, and he could only watch as the Armenian's left-footed effort flew past the helpless Pickford, whose best efforts were again being wasted.
The Black Cats looked a beaten team.
United were playing well below their best tempo but encountered little resistance. Defoe fired a free-kick on target, but Paul Pogba blazed over a major opportunity minutes later as a reminder of their numerical dominance and their superior speed on the break.
Defoe kept trying to force the issue, he and Didier Ndong again Sunderland's brightest attackers.
The Gabonese midfielder played plenty of slack passes but at least showed a willingness to show for the ball, to drive at the United defence and try and create openings for his team.
He found Defoe on the break and the 34-year-old did brilliantly to get his shot away; Romero looked beaten but it whistled agonizingly wide.
Moyes threw on Fabio Borini in a hunt for goals, the visitors regularly finding themselves two on one as they entered the Black Cats half. Only their profligacy and perhaps complacency on the break prevented them from running clear, Zlatan and Marcus Rashford failing to find the final ball.
The teams continued to trade attacks, Pogba flashing wide with a free-kick, Defoe just unable to convert Anichebe's cross, but both were playing in a manner that reflected the sad truth for the flat home support.
This game, and this season, was as good as done.
Rashford's goal, another firm finish, was merely another painful footnote.