Giuseppe Rossi suffered what appears to be another serious right knee injury on Sunday in Fiorentina's 1-0 win at home to Livorno, his club have confirmed.
Rossi was hurt by a a tackle by Leandro Rinaudo and limped off the pitch in obvious pain in the 71st minute.
A statement on the club's official website read: "ACF Fiorentina announce that following a challenge from Livorno player Leandro Rinaudo, Giuseppe Rossi suffered a sprain to the knee that had already been operated on. Tomorrow more tests will be performed."
The New Jersey-born Rossi first tore the anterior cruciate ligament in his right knee in 2011, and again in a training session six months later before he had returned to first-team football. He only began training again with Fiorentina again in March 2013.
“We are all worried about Rossi," coach Vincenzo Montella told Sky Sport Italia. “We are above all sad for his state of mind. It’s a real shame, as the foul seemed to be intentional and the player behind it didn’t even apologise. Usually when someone commits a foul like that you say sorry, as there was no way Rinaudo was going to get that ball.”
Rossi has been in fine form this season and is the top scorer in Serie A with 14 goals.
The 26-year-old's performances had left Fiorentina fans dreaming of Champions League football again next season.
"You can see that Rossi is a genius footballer," Montella told Italy's Mediaset television of the Italy international last week. "He's also a really nice lad, open to everybody. He may arrive for training a bit late sometimes, and it's not like I'm delighted about that, but I do turn a blind eye now and again."
Another ACL injury would significantly jeopardise Rossi's hopes to play in this summer's World Cup, where Italy will face England, Uruguay and Costa Rica in the group stage.