Danny Welbeck's match-winning return to Old Trafford was a triumph of "mental strength", according to his manager.
Arsene Wenger chose Welbeck as his central striker ahead of in-form Olivier Giroud, and the former Manchester United man responded by knocking his old club out of the FA Cup with the winner in a 2-1 quarter-final victory.
Louis van Gaal sanctioned Welbeck's sale in the summer and stood by his decision in the aftermath of a defeat that leaves United facing a second straight season without a trophy.
And it was left to Wenger to speak up for the England international as he said: "We want all to do well when we play against our former club, to show we are a great player, but it's not easy to deal with the mental situation.
"He focused just on his game and didn't do anything special, just played football like you want him to play, and I think he has shown mental strength."
The intensity of the game at times recalled some of the club's famous meetings at the turn of the century, and United collected seven bookings, two of them for the sent-off Angel Di Maria.
Wenger said: "On the day we have shown we were mentally at the level requested, because it was a very physical, committed game where we needed to deal with a very decided Man United team.
"We did that well without forgetting to play our football. You need some quality to do that. I feel we played a good game from the start and we deserved to win the game."
Arsenal's reward for their win is a Wembley semi-final against either Reading or Bradford, who beat Arsenal in the League Cup in December 2012.
Wenger's side struggled at the same stage against Wigan last season before winning on penalties and going on to beat Hull in the final.
He said: "Last year we played a tricky game at Wembley in the semi-final and you expect that again,"