Jan.29 (GMM) Wednesday marks exactly one month that Michael Schumacher has been in a coma.

Doctors at the Grenoble hospital have kept the great German unconscious through anaesthesia, and he is now listed in a "stable" condition.

But the length of 45-year-old Schumacher's coma has neurosurgeons all around the world concerned.

"Regarding neurological functionality, the more time that passes, the more the ability to recover can be challenged," Lyon neurosurgeon chief Patrick Mertens told France's RMC Sport.

"It is certain that in the weeks to come, if we then come to two months and there is no improvement, you worry even more about the ability to recover," he added.

However, another French-speaking neurosurgeon told La Presse newspaper that he has seen "patients improve between one and three years after an accident".

But Gerard Audibert, a neurosurgeon at France's Nancy hospital, told La Depeche newspaper: "The average duration of an artificial coma with a severe brain injury is 15 days."

World champion Sebastian Vettel, once dubbed 'Baby Schumi' by the German press, said he has stayed away from his friend and mentor's bedside since the December accident.

"It's not about me at all it's about Michael only and his family," the Red Bull driver said at the Jerez test on Tuesday.

"I am still as shocked as everybody else and I pray and hope he will come back and the miracle will happen and he will be the person he always has been before," Vettel added.