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.
Luke Shaw and Marcos Rojo have also been purchased for over £40 million as United look to bounce back from last season’s seventh-placed finish but 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 are interested in Real Madrid winger Angel Di Maria, who was excluded from the club’s squad as they were beaten 1-0 by Atletico Madrid in Friday’s Supercopa.
Reports suggest a deal worth over £50m is imminent but Van Gaal insists he is keen to further 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.”