The French defender, on loan from Liverpool, suffered a knee injury in tonight's 1-0 home loss to Tottenham Hotspur and according to the manager, could quite possibly miss the remainder of the season.
"He has hyperextended the knee," said Allardyce.
"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."
Sakho has been impressive since making the move to South London and his absence is likely to affect the Eagles' push for a top-half finish.
Need a VPN? Or seeking to switch? Then support Tribal Football by clicking here to sign up with our official partners NordVPN. Not only will you receive the best VPN experience, but also great discounts by signing up via Tribal Football. Thanks for your support!