The 45-year-old is being gradually woken up from an induced coma following five weeks in hospital.
The seven-time F1 champion has undergone two operations on his brain since hitting his head on a rock while skiing off-piste in the French Alps on December 29.
"Waking from a coma is not like how it is portrayed in the movies," IB Times quotes Gregg as saying. "It can be a very gradual process that can take several days or weeks.
"For the family, the initial fear about whether or not the individual will survive is replaced by fear of what the future will hold and what level of recovery their loved one will make. Put simply, the effects of brain injury can be devastating and last a lifetime."
Schumacher has received hoards of tributes from figures within F1 since suffering the accident.