Depay joined United from Dutch champions PSV in a £25 million ($38 million, 33.6 million euros) transfer on Thursday on a reported four-year contract.
Van Gaal said he broke with his own usual transfer protocol to conclude the deal, which will be formally completed when Depay undergoes a medical next week, in order to head off interest from PSG.
“I have been forced to sign him before the end of this season,” van Gaal told a news conference on Friday.
“We have to focus on the remaining games but when you sign a new player you can disturb the focus of the current ones,” the Dutch manager added.
“But because of the market, we had to handle this because otherwise he would have signed for Paris Saint-Germain,” he added.
“At that time he was not reached by me. But because of the club’s relationship with PSV, I could handle it,” the former Netherlands national team boss explained, with United having signed Jaap Stam and Ruud van Nistelrooy from the Dutch giants in the past.
Meanwhile United’s hopes of securing a top-four finish and Champions League football were dealt a blow when van Gaal said influential midfielder Michael Carrick will not play again this season.
As recently as last week, van Gaal was claiming he fully expected the England international to play a part in the closing stages of a campaign which could yet result in frustration were his team to lose out to bitter rivals Liverpool in the chase for a top-four place.
But van Gaal revealed on Friday that Carrick’s calf injury, sustained in the derby victory over Manchester City last month, was worse than first thought.
“Carrick is not available, it was more serious than we thought,” van Gaal told a news conference.
“We tried everything but now we have to think about next season. He shall not play anymore this season, it’s better now to focus for him on next season and being fit to start.”
– Selection not balanced –
The United manager, whose side are away to Crystal Palace on Saturday, said Carrick’s absence would hurt his side.
“When you see the stats, it’s clear that he’s very important but that’s also because we have only right-sided holding midfielders.
“That’s what I have been saying already in the whole of the season, that our selection is not balanced, and I want to restore that in the next transfer period,” he added.
“Life is not ending at 34, but for a professionalfootball player, it’s close by. That’s why I have to handle again in that position,” van Gaal explained.
The value of the 33-year-old Carrick to United has been particularly evident in the games since the derby win which have seen United lose consecutive matches to Chelsea, Everton and West Bromwich Albion without him in the line-up.
Van Gaal, meanwhile, congratulated Chelsea and their manager, Jose Mourinho, for winning the Premier League title and refuted the claim from some quarters that the wealthy London club were not worthy champions.
“I have sent him (Mourinho) a message but I don’t know if he has received it because he is always changing his number,” joked van Gaal, who had Mourinho as a member of his staff when he was coach of Barcelona.
“But I want to wish Jose Mourinho my congratulations for winning the championship.
“It is well deserved, I think. When you look at the statistics, you cannot doubt that they are deserved champions,” he insisted. – Agence France-Presse