The F1 racing driver emerged from his coma on 16 June, yet numerous top medical practitioners have said since that his prognosis might not be as significant as some may have thought.
"He will remain an invalid all his life and will always remain dependent on others' help," said neurology specialist Riederer.
He added that Schumacher will have "permanent" damage and that it will be a "success" if he manages to sit up unaided within the next three months and operate an electric wheelchair within six months.
However, he said that his coma awakening was still "positive".
"It is an incredibly positive message when someone wakes up after being in a coma for half a year," he continued.
"For him it is positive, but also for his family, this is hugely important."
Earlier this week, Schumacher's former doctor, Gary Hartstein, warned that the former F1 champion could be in a "minimally conscious state".
