He said: “I did not buy a third striker, so Marcus had a chance. He was on the bench at Watford earlier in the season. He was having a very good period.
“He might have come in for that match but we played another system, and Memphis Depay and Jesse Lingard played very well.
“We have a small squad and have no more than two senior players for each position, so you can give an opportunity to a youngster.
“We have done that too much, of course, because at times we have played with five youngsters, and that is too many. But the opportunity is there.”
Van Gaal, meanwhile, has dismissed claims that Rooney needs an operation on his knee – and expects him back in action in around a month.
“He does not need an operation,” the manager said. “He may even be back in March.”