Florian Lejeune has suffered a "serious" knee injury – just months after returning from surgery.
Lejeune was stretchered off during this afternoon's 1-0 defeat to Crystal Palace after going down following a challenge with Andros Townsend.
The Newcastle United defender – who ruptured the anterior cruciate ligament in his right knee last summer – was applauded off the pitch by fans after injuring his left knee.
Rafa Benitez said: "He seems bad. It's the other knee, but it seems bad, so we'll have to wait and see what the doctor says tomorrow.
"He was here after the game, and I was talking with him. Obviously, he was disappointed, and afterwards the doctor said we need to assess him tomorrow, but it seems bad.
"He says he felt a click in his knee, and that's bad news normally. It's his left knee."
Benitez told BBC Radio Newcastle: "It’s seems serious. It’s the other knee. He had a cruciate ligament problem in the right one, this is the left one. It seems bad, but we have to wait."
Lejeune – who made his first-team comeback in January – left St James's Park wearing a knee brace and with the aid of crutches.
Newcastle had created a number of first-half chances against Palace, who won a penalty nine minutes from time when Wilfried Zaha was brought down by DeAndre Yedlin.
Luka Milivojevic, the visiting team's captain, converted the spot-kick to condemn United to their first Premier League defeat at home in more than three months.
"These are the kind of games that leave you very disappointed," said Benitez, United's manager. "Every game is a final, and every game we will try to get three points.
"It's a game we can't believe we lost. With the chances we had and the control we had, we made too many mistakes in the last minutes. We had enough chances to win the game."