Well this came as a shock.
The consequences of the injuries caused by the bike-accident in February, fractures in the area of head and neck, unfortunately have turned out to be still too severe. That is why my neck cannot stand the extreme stresses caused by Formula 1 yet. This are the clear results of the examinations we did on the course of the past two weeks and the final examination yesterday afternoon. As there were no improvements after the day in Mugello, I decided at short notice on Sunday to do that thorough examination already yesterday.
You can check his entire statement here