The Malaga centre-back has not trained fully over the last couple of days because of the problem, and coach Bert van Marwijk admits he is a likely absentee on Saturday.
"As things stand now, it is 80 per cent certain he will not play against Denmark," said Van Marwijk.
The Oranje boss will consider calling up a replacement if Mathijsen fails to show rapid improvement.
"I will wait and see how things develop over the next two days and what the prospects are," he said. "Then I will decide.
"Muscle injuries are always difficult to predict. Missing the first game of the tournament is different to missing the first two games."