Having been left out of Jurgen Klopp's first team plans at Anfield this season, Sakho was granted a temporary move to Palace in January and he has quickly established himself as a key performer.
And Allardyce is keen to discover the extent of the issue, acknowledging that this blow could see the defender return to Liverpool without playing again for the Selhurst Park club.
"Our biggest problem is at centre half - Sakho with another knee injury, but it is a good job we are blessed with outstanding centre halves," Allardyce told Sky Sports.
"He has hyperextended the knee. It is certainly serious enough to keep him out for who knows how long, we won't know until we diagnose it.
"But, if it is ligament, it may be the end [of his loan spell], it may not."
Allardyce had previously admitted that it would be difficult for Palace to negotiate a permanent transfer for Sakho at the end of the season.