Hoffenheim midfielder Boris Vukcevic had "low blood sugar levels'' leading to a possible collapse at the time of his car crash, according to a doctor's report.
Vukcevic, 22, was hospitalised after he crashed into a lorry on his way to training a fortnight ago, leaving him in need of two operations before being put into an induced coma.
A statement on the club's official website said that tests carried out on the Germany Under-21 international, who currently remains in a critical but stable condition, suggests that he may have collapsed at the wheel of his car.
"We remain in close contact with the doctors treating him, and with Boris' family,'' Hoffenheim's general manager Andreas Muller said.
"The situation remains very saddening, but we remain very optimistic that he will make it.''
Vukcevic has featured in every match for Hoffenheim this season, while in total he has played 78 Bundesliga matches for the club, scoring six goals.