Newcastle have confirmed that Steven Taylor will undergo Achilles surgery on Monday and is likely to be sidelined for the remainder of the season.
The centre-back limped off during the closing stages as the Magpies went down 3-0 at home to Chelsea in the Premier League on Saturday.
Alan Pardew admitted after the final whistle that he feared it could be a serious problem, and scans have now revealed the extent of the damage.
Taylor will now be sent for an operation that will leave him facing a lengthy spell of rehabilitation.
A statement on the club's official website read: "Steven Taylor sustained an Achilles injury yesterday at the end of our game with Chelsea.
"After medical assessment he will be having surgery tomorrow and it is likely he will be out for the rest of the season."
The news, while not unexpected, will come as a huge blow to Pardew, who also saw skipper and Taylor's central defensive partner Fabricio Coloccini hobble off against Chelsea with a thigh strain.
Coloccini will be a doubt for next Saturday's trip to Norwich in which James Perch, who replaced him against Chelsea, is certain to start.
Pardew has been able to field the same back five in each of the Magpies' 14 games to date this season, and that has proved a significant factor in the club's fine start.
However, there was always an awareness that, with Mike Williamson still working his way back to fitness after a broken arm and an ankle problem, there was a lack of depth in the defensive area.
That absence of genuine cover has now been exposed and Pardew's squad will be tested to the full as they attempt to negotiate the next few weeks until the manager can attempt to address the situation during the winter transfer window.
Taylor will be a particular miss after proving one of the club's key men in their recent revival.
Greenwich-born, but Tyneside-raised, he has established himself as one of the first names on the team-sheet with a consistency which has led his manager to repeatedly recommend the former England Under-21s captain to senior boss Fabio Capello.