Gary Hartstein, F1 medical delegate until 2012, wrote on his blog that he had learned "from usually impeccable sources who have access to this information" that mistakes had been made in the German great's initial care.
"I think that serious lapses in judgement were evident during Michael's initial management... these lapses could (and almost certainly did) worsen the outcome in Michael's case," he wrote.
Hartstein later clarified on Twitter that he was referring to "botched" pre-hospital care and not standards at the Grenoble hospital where the seven-time world champion is being treated.
But Hartstein did not give details about what mistakes may have affected Schumacher, 45, who is in an induced coma after suffering serious head injuries in a skiing accident on December 29.
Hartstein, in the blog written ahead of this week's Malaysian Grand Prix, said: "As time goes on... it becomes less and less likely that Michael will emerge to any significant extent."
Schumacher's plight hangs heavy over Formula One, with his former team Ferrari this week tweeting a team photo with well wishes and the message "Forza Michael."