Napoli released a statement confirming the injury to defender Juan Jesus, so with Alessandro Buongiorno also out, they must turn to Rafa Marin.
There had already been reports that the Brazilian centre-back had sustained a potentially season-ending setback during the 3-0 victory over Empoli in Serie A on Monday evening.
Juan Jesus had tests today at the Pineta Grande Hospital, which showed a lesion to the semimembranosus muscle in the right thigh.
Although the statement does not include a prediction for how long he will be out for, this is effectively a torn hamstring and will need at least a month out.
Juan Jesus season is likely over for Napoli
The 33-year-old made 17 competitive appearances for Napoli this season, contributing one goal and one assist from defence.
It is a problem for Napoli, who are already missing Buongiorno in the same role with an adductor injury.
Antonio Conte must therefore turn to Rafa Marin to receive his first start against Monza in the next Serie A round.
Remarkably, Marin received only 23 minutes of Serie A football since joining from Real Madrid last summer in a complicated loan deal.
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