Louis van Gaal found it impossible to defend Angel di Maria after the Argentina international was sent off for grabbing the back of referee Michael Oliver's shirt in Manchester United's FA Cup quarter-final defeat to Arsenal.
Van Gaal's dream of landing a trophy in his first season at Old Trafford went up in smoke as United lost 2-1 to the Gunners on Monday night.
The hosts ended the game with 10 men after the latest episode in Di Maria's disappointing Red Devils career unfolded in the 77th minute.
After being cautioned for simulation, Di Maria remonstrated with referee Oliver and then grabbed his shirt and yelled at the official when he had turned his back on him.
Manchester United 1 Arsenal 2 player ratings
Manchester United 1 Arsenal 2 player ratings
1/24 David De Gea
Without him United could have been humiliated. A save from Cazorla seemed frankly impossible but the Spaniard made it look merely ordinary. 8
2/24 Antonio Valencia
A horror show from the Ecuadorian. Woeful defending allowed Monreal through to score and his back pass then allowed Welbeck through for the second. 3
3/24 Chris Smalling
He never seems to look comfortable in the United defence. His loose pass early on to Sanchez summed up yet another careless performance. 5
4/24 Marcos Rojo
The least poor of a dreadful performance by the United defence, he seemed to be nervy around Welbeck, particularly when playing at centre-back. 5
5/24 Luke Shaw
Struggled to deal with the direct running of Oxlade-Chamberlain whilst also offering little of his usual attacking skill. Unsurprisingly he lasted just 45 minutes. 4
6/24 Daley Blind
Often found himself overrun as he looked to protect a leaky defence on his own, his role in the game diminished after Carrick’s arrival. 5
7/24 Ander Herrera
Made good runs early on, Fellaini couldn’t find him. Unfortunately he was hauled off at half-time as his colleagues gave him few opportunities. 5
8/24 Angel Di Maria
His best game for some time, with his left-foot deliveries causing chaos for Mertesacker. Then he ruined it all by grabbing the referee’s shirt. 5
9/24 Marouane Fellaini
Another big-money signing who shone, his physicality caused Arsenal’s defence all sorts of panic. But as a forward he lacks any real confidence in front of goal. 6
10/24 Ashley Young
United’s star outfield player’s contest with Bellerin was thrilling in which the Englishman often got the better, though his final pass was lacking. 7
11/24 Wayne Rooney
Oozed confidence throughout, scoring a wonderful header and running powerfully at the Arsenal defence with menace. 7
12/24 Best off the bench: Michael Carrick
His passing radar kept United ticking and allowed them to dominate possession, even if that wasn’t enough to carry his side through. 6
13/24 Wojciech Szczesny
A dropped cross in the first half could have been calamitous as throughout the match he seemed to be lacking the confidence usually associated with him. 5
14/24 Hector Bellerin
An early yellow card put a target on his back but he continued in his usual fashion, a pacey presence in attack and defence. 6
15/24 Per Mertesacker
Implausibly gave away a corner early on as nerves looked to be getting the better of him, he struggled aerially against Fellaini and Rooney. 5
16/24 Laurent Koscielny
The Frenchman brought invaluable composure to the Arsenal defence, the rarity with which he ever had to step in a testament to his organisational skills. 7
17/24 Nacho Monreal
Exceptionally threatening going forward, he picked up a well-deserved goal. But the Spaniard was too timid around Di Maria, giving him space to cross. 6
18/24 Francis Coquelin
A quiet game from the powerful Frenchman but that is just what Wenger would have wanted. His tackling continues to be ferocious. 6
19/24 Santi Cazorla
His range of passing was vital to Arsenal’s counters and he even displayed his recently discovered tackling ability when the visitors needed to defend. 7
20/24 Alex Oxlade-Chamberlain
Enjoyed his duel with Shaw and his powerful running created Arsenal’s first. The visitors missed his explosiveness after he went off injured. 7
21/24 Mesut Ozil
Peppered the contest with moments of pure class, his understanding of space and his colleagues’ movements brightened Arsenal’s play and found their first goal. 8
22/24 Alexis Sanchez
As ever the Chilean was full of energy and menace, his passes always seeming to put Arsenal in a more dangerous attacking position. 8
23/24 Danny Welbeck
He even managed to make an open goal look difficult but nevertheless he got his goal and his energy caused problems for Rojo. 7
24/24 Best off the bench: Aaron Ramsey
His driving runs from deep gave United’s defence another problem to deal with. His presence on defence was invaluable when United chased an equaliser. 7
Replays showed the first booking on Di Maria may have been harsh and Aaron Ramsey had pulled the player back, but the £59.7million man's actions thereafter deserved sanction, according to his own manager.
"I think he's touched the referee and that's forbidden in every country, so he has no excuses," the United manager said.
"In Spain he knows that he doesn't touch the referee, but that is also in his emotion.
"I've already spoken with him, he knows my opinion but also I have to see on the video.
"He doesn't have to touch the referee. That is not so smart of him."
Di Maria actually started the game reasonably well, setting up Wayne Rooney for an equaliser with a wonderful curling cross.
But his dismissal clearly angered his manager and the player himself, who refused requests for an interview post-match.
When asked whether he would discipline Di Maria, Van Gaal said: "No, I don't think so."
The Dutchman admitted United handed Arsenal victory on a plate with the second goal.
Antonio Valencia, who had been the best United player on the pitch, according to Van Gaal, sold David de Gea short with a poor back-pass, leaving Danny Welbeck to knock the ball past the goalkeeper and into an empty net.
"I think it is very disappointing when you give the victory (away)," Van Gaal said.
"You doesn't lose (because of) your opponent - you have lost (because of) yourself. That is very disappointing."
When asked whether he stood by his decision to sell Welbeck and stick with Wayne Rooney, Robin van Persie and Radamel Falcao last summer, the reply was emphatic.
"Yes," Van Gaal said bluntly.
"A lot of times against the club who a player was playing for (he) shall be very motivated, but I think we gave that goal (away)."
When it was put to him that Falcao, who did not make it off the bench, had flopped this season, Van Gaal fired another retort back.
"He has scored four goals and (has made) three or four assists," he said. "And he stimulates also the other players."
Arsene Wenger said he expected Welbeck to be fired up for the game, given that he was facing his old club.
"We want all to do well when we play against our former club to show we are great players," the Arsenal manager said.
"It is not easy to deal with the mental aspect of it."
Arsenal will face either Reading or Bradford in the semi-finals.