Dr Gary Hartstein, who worked with the German racing driver during his time as rescue co-ordinator at the championship, said he could be in a “minimally conscious state”.
Fans around the world rejoiced at news that he was out of his five-month coma on Monday.
His manager, Sabine Kehm, said Schumacher, 45, would “continue his long phase of rehabilitation” but offered no further information on his condition.
But Dr Hartstein said announcements in April about “moments of consciousness and awakening” meant that by definition he was already out of the artificially-induced coma and the language used to describe his condition had merely been changed.
In an article on his blog, he speculated that Schumacher had gone beyond his minimally conscious state in April, more details about his rehabilitation and recovery would have been given.
Dr Hartstein wrote that the recent news still left "a huge space of sadness for Michael’s family, and for you, his fans.”
The seven-time Formula 1 champion left the hospital in Grenoble in the French Alps where he has been treated since he suffered serious head injuries in a skiing accident on 29 December.
He has been transferred to the Vaud university hospital at Lausanne, in Switzerland, close to his family home.
French medical experts have also warned that only one in 10 patients who spend the length of time in a coma that Mr Schumacher has recover most of their mental and physical capabilities.
His family released a statement on Tuesday thanking fans for their support.