De Guzman was included in Netherlands' final FIFA World Cup party of 23, confirmed on Saturday, but his place looks less assured after it emerged on Monday he had pulled up in training.
The finals in Brazil would be the 26-year-old's first World Cup, and Van Gaal is hopeful that the injury is not too serious.
"He felt something in his hamstrings and therefore I stopped the training session as we were playing 11 v 11," he said.
"Now we have to wait and see in the coming days whether it is bad or not so bad.
"Hopefully it's a light injury. We will wait a couple of days and then we will be able to say what it is.
"De Guzman has earned his position in the team.
"He had a rather good game against Ghana on Saturday. I liked him. I said it to him. So now I have to adapt and play another player in his position which is always not so good."
