Ter Stegen joined Girona on-loan from Barcelona in January with the aim of being fit for World Cup selection. However, the veteran goalkeeper suffered a serious hamstring injury in only his second appearance.
"It's obviously an extremely tragic situation for Marc; it would have been his turn now," Nagelsmann told Kicker.
"That something so dramatic would come from such a simple action as a normal pass. Looking at his history with the national team, it would have been his turn now."
Without completely closing the door, Nagelsmann did make it clear that the schedule is tight.
"The book isn't completely closed, but there's no doubt it's going to be very tight, because we have to take into account not only the recent downtime, but also the previous period," said Nagelsmann.
"He's barely played for a year; that's extremely little."