As is much of the world at present, he is watching with keen interest the progress in Grenoble of injured seven time world champion Schumacher, who is in a critical condition after a skiing crash last Sunday.
Friday, Schumacher's 45th birthday, is the sixth day since the accident, but Kehler told Bild newspaper that the great German's family, friends and fans face a longer wait.
"Generally, it takes two to three weeks until a patient with such a severe trauma can be woken up," he said.
"But it can take days or even weeks until the patient opens his eyes. Unfortunately, it is also possible that the person does not wake up properly."
Professor Kehler said it is a good sign that Schumacher, although critical, has entered a period of stability.
"In a severe craniocerebral trauma, the first hours and days are crucial to see if the pressure and swelling continues to increase. Especially critical are the first three to four days.
"When patients get through those first few days, everyone can breathe a little. But no statement can be made yet about the patient's survival or the outcome.
"If the patient continues to remain stable, you can shut down the measures to reduce intracranial pressure and then dissolve the coma," he explained.
A Paris neurosurgeon, Philippe Decq, told France's RMC Sport that Schumacher will then reach a crucial point in his recovery.
"After a severe traumatic brain injury, if three weeks passes and there are no signs of awakening, then from a prognostic point of view it is very bad," he said.
Schumacher's manager Sabine Kehm implied by text message on Thursday that the former Ferrari and Mercedes driver remains in a stable yet critical condition.
"We will not make statements unless there is something new (to report)," she said.
A statement issued by Schumacher's family reads: "We all know he is a fighter and will not give up."