However, Van Gaal's first competitive match in charge ended in disappointment, with Swansea City running out 2-1 winners at Old Trafford in their Premier League opener on Saturday.
"This is not going to be a quick fix," he told talkSPORT.
"They have still got two weeks to go in this transfer window. They definitely need to maximise the opportunities in this window and probably do something in January as well. This process will take a long, long time.
"We need to strengthen at the back, we need to strengthen in midfield and that is foremost the two areas they need to look at. In my opinion, they need to bring in at least four top players to crank up the level."