Manchester United boss Louis van Gaal admits they took advantage of Arsenal's sluggish defence to win on Saturday.
Per Mertesacker and the rest of the Arsenal defence were left stranded upfield for Wayne Rooney's winner.
"I was sure that Arsenal wants to attack and to press us," said a delighted Van Gaal.
"Then, you know that Arsenal is giving a lot of space away and then Mertesacker and our friend Monreal has to defend.
"That's why I put Di Maria against Mertesacker and Robin van Persie against Monreal.
"That's why I changed van Persie for James Wilson. More pace! So, yeah, that's what you are thinking in advance of a match and when it is ending like this you can be happy."
