Although the news was worrying, there was widespread relief that the damage was not to the anterior cruciate ligament, which has twice been subject to surgery.
The club did not say how long the Italy striker, who has scored 14 league goals in his comeback this season, would be sidelined.
"For a correct valuation of the extent of the damage and the outlook, a visit to a specialist will take place in the next few days," Fiorentina said in a statement.
Fiorentina added that he would be treated by renowned U.S. knee specialist Richard Steadman.
Some Italian media reports said Rossi would be sidelined for around three months and Gazzetta dello Sport said his place at the World Cup in Brazil was at risk.
"We are waiting for you," Gazzetta quoted Italy coach Cesare Prandelli as saying. "He has such immense strength of character, he knows full well how much we admire him."
Rossi limped out of the game in the 71st minute following a rough tackle by Livorno defender Leandro Rinaudo, who was booked.
Fiorentina angrily criticised Rinaudo, who argued with the referee and his opponents and did not apologise to Rossi.
"To me it was a normal challenge," Rinaudo told reporters after the game. "There is attention because it involves a player such as Rossi.
"I did not apologise because I was immediately surrounded by some of Fiorentina's players. But, I will call him to do so."
American-born Rossi's troubles began when he suffered an anterior cruciate ligament injury while playing for Villarreal in a Spanish league match against Real Madrid in 2011.
He fought desperately to be fit again in time Euro 2012 only to re-injure the knee in training the following April, causing him to miss the tournament.
Villarreal sold him to Fiorentina at the start of this year and he finally made his comeback against Pescara in May, playing 27 minutes in the final match of last season.
He has played for Italy four times this season and scored his first international goal since his comeback in the 2-2 draw against Nigeria in London in November.
(Reporting by Brian Homewood, editing by Tony Jimenez and Justin Palmer)
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