The Old Trafford club has already acquired Ander Herrera, Luke Shaw and Marcos Rojo this summer but the Dutchman has hinted that more signings could still be made.
The Red Devils kicked off their season with a shock 2-1 home defeat to Swansea, despite the presence of new signing Ander Herrera from Athletic Bilbao.
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Luke Shaw and Marcos Rojo have also been purchased for over 55 million euros as United looks to bounce back from last season's seventh-placed finish, and Van Gaal says he has not yet finished his summer recruitment drive.
"You have to buy what can improve your team," the Dutchman told The Telegraph. "That's why I bought Rojo, because he can play here and here [in two positions]. So for us he is very good.
"No, no, we shall buy maybe more players. You never know."
United is interested in Real Madrid winger Angel Di Maria, who was excluded from the club's squad as it was beaten 1-0 by Atletico Madrid in Friday's Supercopa.
Reports suggest a deal worth over €60 million is imminent but Van Gaal insists he is keen to assess every member of his squad before investing further.
"I don't think it is fair for me to come here and fire all the players," he added. "So I give them a fair chance. I think.
"Now [after judging the players] you can say something."