The 45-year-old has been in an induced coma in Grenoble after he hit his head on a rock while skiing off-piste in the French Alps on December 29.
"Michael, friend and brother in arms, even after so many tests and races, even when I though I finally knew you, you were always able to surprise me in some way, thanks you your amazing abilities as a driver and as a man," the Daily Star quotes Stella as saying.
"You proved you could always raise the bar that little bit further, without ever giving up. We fully expect you to surprise us once again with this new challenge you are facing. You are in our thoughts with all our love."
Reports have emerged claiming that the seven-time Formula 1 champion is undergoing muscle training to prevent his body from seizing up while in a coma.