The French defender was replaced at half-time by Ryan Nelsen in Sunday's 3-0 Premier League defeat at Everton.
Allardyce says the club are in the dark over the exact injury the former Marseille stopper suffered.
However, he fears it could be ligament related as the knock was not inflicted by a Toffees player.
"He fell on his knee, we aren't sure of the extent of the damage yet," he told the club's official website.
"But it is a little worrying that is it isn't a kick so he may have twisted or tweaked a ligament."
Blackburn 6/5, Nottm. Forest 15/8, Draw 12/5