Manchester United were forced to act quick in agreeing to a transfer for Memphis Depay before the summer window.
Paris Saint-Germain were expected to get involved in the bidding.
Depay was bought by United from PSV for £25 million.
“I have been forced to sign him. I don’t want to sign him before the end of the season,” said Van Gaal. “We have to focus on our games. When you sign a player you disturb the focus of your present group of players.
“The coming matches are very important for Manchester United and for all of the players. But because of the market, I have to handle – I must handle it – because otherwise he would have signed for PSG. That is why I signed Memphis.”