Dr Helmut Marko says grands prix are not the same in the absence of Niki Lauda.
Formula 1 legend Lauda is currently recovering from a lung transplant in a Vienna hospital.
"We all know that it was very serious and it still has not been said that the lungs are rejected, but that's something private I don't want to comment on.
He needs our support and we know he's a great fighter. I have to say that there were two grands prix without him and it's not the same," Marko, an Austrian and former F1 driver like Mercedes team chairman Lauda, added.
"Please understand that we must maintain the privacy of our patients. We do not comment on rumours," the Vienna hospital where 69-year-old Lauda is recuperating said.
