"Bastian is the heart of the team, the motor in our midfield. Of course it would be a big disadvantage if he could not play," said Low.
The 25-year-old picked up a thigh muscle injury in their final Group D match against Ghana and will miss training on Friday for the second day running.
"There is a huge question mark for both players. Their muscle problems have not yet healed and they will not train today," said the 50-year-old coach.
"It is very questionable whether they will be 100 percent fit because we need players who are fully fit for this game. With a 90 percent fitness it is just not possible."
"I can well imagine that Toni Kroos could come in," he said. "He came on against Ghana and immediately found his way into the match. He was not nervous at all."
Left-back Holger Badstuber, who had started in Germany's two other Group D encounters against Australia and Serbia, is expected to replace Boateng.
