The Colombian was not in Van Gaal's matchday squad for the 1-0 Premier League loss at Old Trafford as Van Gaal's strikeforce failed to muster a shot on target.
United were downed by a second-half Dusan Tadic strike with Southampton moving third in the Premier League as a result of the win but Van Gaal stuck by his squad selection post-match.
Asked by reporters whether he regretted the decision, he explained: "No. The reason is that I have to see and compare players in my selection, that is what I'm doing when we train.
"Then I have to look at the composition of my selection and what I can do during the match. What players I have and I want to play with those kind of players, did not have match rhythm.
"So I had to change for example, Shaw and Di Maria, I know that in advance. Normally, I would have changed Di Maria for [James] Wilson because I also need speed in attack.
"I know in advance I have to change Luke Shaw and normally I have to change Daley Blind because he also needs match rhythm.
"In advance I know I have to change defensive players, when you see the composition of the bench and I want to change the playing style, which I did with [Marouane] Fellaini, then I don't have any position for a second striker.
"This is this moment, next week is another moment. Shall I decide another way?"
