Louis van Gaal admitted he had not prepared for the possibility of matchwinner Danny Welbeck facing his former club at Old Trafford. The Manchester United manager, under pressure after this dramatic 2-1 defeat in the FA Cup quarter-final, expected Olivier Giroud to spearhead the Arsenal attack. Instead Welbeck scored Arsenal's glorious 61st minute winner when he clipped the ball beyond David de Gea and ran on to score in front of 9,000 travelling Arsenal supporters. Danny Welbeck slides the ball into an empty net to secure a 2-1 win for Arsenal against Manchester United . The former United striker celebrates in front of Arsenal's 9,000 travelling supporters at Old Trafford . Van Gaal, who sold Welbeck to Arsenal in the summer because he didn't believe he was good enough, said: 'In football it happens, but it was also a surprise that Wenger put him in that position. 'When a player returns to play his former club he will always be very motivated, but we gave the goal away. 'It is very disappointing because we gave the victory to Arsenal. The players are disappointed because they know it could have been different. 'This is a big blow, we have to recover. We are sportsmen so we have to recover. I cannot say that the motivation of the team was bad. 'We have to show fantastic fighting spirit, that is not the problem.' United finished with ten men after the dismissal of Angel di Maria in the second half. He was booked by referee Michael Oliver - who had an outstanding game - for diving and was then shown another yellow card for tugging at the official's shirt. Van Gaal added: 'Di Maria knows that he doesn't have to touch the referee. That is not so smart of him. Angel di Maria is shown a red card by referee Michael Oliver after picking up two bookings inside a minute . Di Maria heads for the Old Trafford tunnel after a moment of madness cost him a second yellow card . 'But controlled emotion is not so easy in a match like this, but I mentioned it before the game because I know these red cards are easily given. 'I cannot give a comment about the referee and you know why. He knows that he doesn't have to touch the referee. 'We have to see every decision of the referee in the circumstances because he whistles, but I want to see the video tape to see how he has decided it all.' Arsenal took the lead here when Nacho Monreal ended a brilliant move by Wenger's side in the 25th minute, but Wayne Rooney equalised four minutes later. The FA Cup holders, who will now play Bradford or Reading in the FA Cup semi-final, went on to score again when Welbeck read a disastrous backpass by Antonio Valencia. Welbeck barely celebrated his goal, his eighth for Arsenal since his move from United, but Wenger insisted he had added motivation to score. The Arsenal manager said: 'He has pace, he also has a bit of psychological consideration by playing against his old club, and I have to rotate a bit without affecting the players. Louis van Gaal and his assistant Ryan Giggs gesture on the touchline during United's defeat by Arsenal . Van Gaal opted to sell matchwinner Welbeck to Arsenal for £16million in last summer's transfer window . 'I thought we might need to counter attack, so I played him. He has had a great game. 'We want everyone to do well against their former club to show they are great players, but it is not aways easy to deal mentally with it. 'I only knew very late that he would play, but he has shown great mental strength. 'He just wants to win, he is a team player. He just wants to do well and he wants to produce s complete performance. 'He kept calm and De Gea made one or two outstanding saves.' Arsenal deserved to win and Wenger insisted that his team can respond to the challenge ahead as they target a sixth FA Cup since he became manager of the club. He also paid tribute to the 9,000 travelling Arsenal fans who made their way to Old Trafford on a Monday evening for this FA Cup quarter-final. Some of Arsenal's 9,000 travelling fans celebrate at Old Trafford after the final whistle . Van Gaal trudges towards the tunnel after seeing his side miss out on a place in the FA Cup semi-finals . He said: 'We started at a high tempo and without apprehension. We were in control. We could have scored more. 'The team was a bit down at 1-1 at half-time, because we felt that we should be winning. 'Mentally we were at the level I requested. It was a physical game, a committed game. We did that well without forgetting to play our football. 'Every win makes us stronger. Winning at Man City earlier in the season helped us to show that we can win big games. 'I have huge respect for the fans to come here. To come here on a Monday night, with 9,000 fans, shows you the huge size of the club. 'We will be favourites in the semi-final, but we have tricky memories from last year. 'It was our hardest game, we but we to focus now on the Premier League and then we have a big game against Monaco in the Champions League.'
Arsenal beat Manchester United 2-1 to reach the FA Cup semi-finals . Goal hero Danny Welbeck left Old Trafford in a £16m deal last summer . Louis van Gaal did not expect his former striker to start on Monday night . Welbeck grabbed the winner and celebrated with the travelling supporters . Angel di Maria was sent off, with the Dutchman admitting that his player was not 'smart' to touch referee Michael Oliver . Van Gaal also described the result as a 'big blow' for his side .