Manchester United manager Louis van Gaal has expressed his disappointed after the Red Devils were knocked out of the FA Cup by Arsenal on Monday night, but was also surprised to see former player Danny Welbeck get the nod ahead of Olivier Giroud.

The Gunners won the game 2-1 making it their first win at Old Trafford since 2006, and the Dutchman has chided his team for handing the victory to their opponents.

He believes that his team had the chances to beat Arsenal in the second half, but it was former player Welbeck, who stepped up and scored the winner following a poor back pass from Antonio Valencia. And the manager was quick to defend his player after acknowledging the obvious mistake made by the Ecuadorian.

"It is very disappointing because we gave the victory to Arsenal. The players are disappointed because they know it could have been different," Van Gaal said, as quoted by the Daily Mail.

"We didn't lose to them, we lost [because of] our own performance, and that's very disappointing. The players are very disappointed; they know it could have been different."

"It's a pity because after we were 1-0 down, we recovered immediately. And the first chances of the second half were for us, so we could have decided the game," the Dutchman explained.

"We gave the ball away in an area where you cannot give the ball away and that's very disappointing. I think the best man in my team made the error and it was decisive. These things happen," the former Bayern Munich manager told United's official site.

Arsene Wenger surprised everyone when he opted to start Welbeck ahead of in-form striker Giroud and the United manager also admitted that this decision from the Frenchman took him by surprise.

"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," the Dutch manager remarked.

The 20-times English champions will now turn their complete attention towards Van Gaal's first priority, which is to finish in the top four and qualify for the Champions League next season and their visitors this weekend are Tottenham Hotspur, who are also in the race for a top-four spot.

"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," Van Gaal concluded.