Valencia struggled at right-back during the clash and Nacho Monreal scored when the Ecuadorian left his position to close down Alex Oxlade-Chamberlain.
Danny Welbeck then capitalised on an awful backpass from Valencia to round Man United keeper David de Gea and score the winner for Arsenal.
It was a poor performance from Valencia to say the least but van Gaal backed him following the game.
Speaking about Valencia, van Gaal said: ”We gave it away [for second goal] but man doing it [Valencia] was best man on my team. Also for him so disappointing. [sic]”
The result leaves United with the very real prospect of once again finishing the season trophy-less.
Their attentions will now turn to finishing in the top four of the Premier League in order to qualify for next season’s Champions League. However, they will need to play much better as they face the likes of Manchester City, Liverpool and Chelsea in the coming months.