HAMBURG, Germany - Brain-damaged ex-Formula 1 driver Michael Schumacher could remain in a coma "for weeks or even months".That's the expert view of Professor Uwe Kehler, head of neurosurgery at the Asklepios Hospital in Hamburg.Kehler is watching events in Grenoble, France, where Schumacher remains in a critical condition after a skiing fall in December 2013.LONG WAIT AHEADJanuary 3 was Schumacher's 45th birthday and the sixth day since he fell and hit his helmeted head on a rock. Kehler told Bild newspaper that family, friends and fans faced a longer wait.Kehler said: "Generally, it takes two to three weeks until a patient with such a severe trauma can be woken. 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."Kehler said it was a good sign that Schumacher, although critical, had entered a period of stability: "In a severe cranio-cerebral 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 everybody can breathe a little but no statement can be made yet about the patient's survival or the outcome."If the patient remains stable measures to reduce intra-cranial pressure can be shut down and the coma dissolved."CRUCIAL POINTA Paris neurosurgeon, Philippe Decq, told France's RMC Sport that Schumacher would 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."Schumacher's manager Sabine Kehm implied by text message that the former Ferrari and Mercedes driver remained in a stable yet critical condition. "We will not make statements unless there is something new (to report)."A statement issued by Schumacher's family read: "We all know he is a fighter and will not give up."