Jesus Gamez suffered a suspected broken collarbone in Newcastle United's 1-0 defeat to Blackburn Rovers.
Gamez, making his full league debut for the club, was stretchered off early in the second half at St James's Park.
The 31-year-old, signed from Atletico Madrid in the summer, had only just returned from a previous spell on the sidelines with an Achilles problem.
United manager Rafa Benitez said: "It's his clavicle. I think it will be bad."