Kalidou Koulibaly (L) has played over 300 times for Napoli since signing from Genk in 2014
Kalidou Koulibaly tore his hamstring during Napoli's collapse at Sassuolo, the Serie A leaders said on Thursday as they deal with a growing injury crisis.
Senegal captain Koulibaly left the field with 10 minutes remaining and Napoli 2-1 ahead at the Mapei Stadium on Wednesday night, one of three players to be forced off with injuries.
A last minute header from Gian Marco Ferrari snatched a draw for the hosts after being two goals down, and Napoli coach Luciano Spalletti now has the spine of his team out for key matches, with star striker Victor Osimhen and midfielder Andre-Frank Zambo Anguissa already unavailable.
Napoli did not say how long their defensive lynchpin would be out for with his "grade two hamstring tear" but Italian media report at least a month on the sidelines.
That would mean missing Saturday's home league clash with high-flying Atalanta on Saturday, Thursday's visit of Leicester City which will decide whether they qualify for the Europa League last 32, and the trip to AC Milan later in the month.
Milan are just one point behind Napoli in a tight title tussle, with Inter Milan a further point back in third and fourth-placed Atalanta only five away.
Some good news for Spalletti comes in the shape of Lorenzo Insigne and Fabian Ruiz, who were also taken off on Wednesday with respective calf and thigh problems.
Napoli said in their statement that "Fabian and Insigne will be assessed once again ahead of Saturday's game".