Darmian, who struggled during the clash, was hauled off at the break at St Mary’s.
He was replaced by Antonio Valencia and the Ecuadorian put in a solid shift as Man United secured all three points.
Van Gaal revealed he thought that Darmian could not handle the pace and trickery of Dusan Tadic on Southampton’s left hand side.
“He [Darmian] gives too much space on Tadic in my opinion and Tadic was the most dangerous man,” van Gaal said.
“That is why I have changed him and Valencia did very well – I didn’t see Tadic anymore in the second half.”
Eyebrows were raised when Valencia was brought on but the change worked well and eventually contributed to the positive result. Darmian has made a superb start to life at United but the games have been coming thick and fast and he may need a rest. Valencia comes in for some flak from United fans but he is a more than capable backup and a loyal servant to the club.