Jesus Gamez will be out until the New Year after suffering a broken collarbone.
Gamez fell awkwardly in Newcastle United's 1-0 defeat to Blackburn Rovers at St James's Park on Saturday.
And the 31-year-old, signed from Atletico Madrid in the summer, needs an operation.
United manager Rafa Benitez said: "We are waiting, because maybe he will need an operation. We have to see.
"Today and maybe tomorrow it’s just to make sure he is fine, because the bone is a little bit high. It’s not serious, but you have to do it just in case it gets worse.
“It’s better than we expected after the game. If he has the operation they have said around six weeks. You never know because each one is different, but more or less this time."
Asked if any other players would miss tomorrow night's EFL Cup quarter-final against Hull City, Benitez added: "We have players with little things, but nothing new.
"We have to manage the players we have now. Sometimes I try to protect the players. We don't take risks."