Rafael Benitez says the injury which Florian Lejeune suffered in Newcastle's defeat by Crystal Palace "looks bad".
The French defender was carried off on a stretcher after 65 minutes after a challenge with Andros Townsend.
"We will have to wait and see what the doctor says tomorrow but it looks bad," Benitez told BBC Sport.
"He had a problem with his right knee at the start of the season but this was his left knee. We don't know."
Lejeune, who missed the first half of the season with a right knee injury, was replaced by Paul Dummett on 65 minutes following a lengthy delay.
The 27-year-old left St James' Park on crutches with his left leg heavily strapped.