Manchester City manager Pep Guardiola has confirmed that midfielder Rodri will be out for "a long, long time" and requires surgery.
The influential Spain international limped off during the first half of the champions' draw with Arsenal in the Premier League at the Etihad Stadium on Sunday.
The 28-year-old is now in Barcelona undergoing assessment and a full prognosis is not yet available.
There have been reports claiming he could miss the rest of the reason but, while the problem is clearly serious, Guardiola remains hopeful it does not prove that bad.
The City manager said: "Still, we don't have the definitive. He'll be out for a long time, a while, but there is some opinions that maybe it will be less than we expect.
"Unfortunately he was injured. Right now, still, I cannot tell you (how long) because we're waiting the last phone calls from him and the doctors to know exactly what, definitely, he has and the type of surgery that he has to get."
The news is a severe blow to City's hopes of winning the title for a record fifth season in succession.
City have not lost any of the last 48 Premier League games in which Rodri has featured.
His only defeat last term in any competition was the FA Cup final loss to Manchester United in May.
He bounced back from that setback to play a key part in Spain's triumph at Euro 2004 and is one of the leading contenders to win this year's Ballon d'Or award.
Guardiola admits he leaves a large void.
He said: "We don't want this, but we will still have a good season. I trust a lot in my players, I have a duty to find a solution.
"Rodri is irreplaceable and the team won't play with the best midfielder in the world for a long, long time. Of course it's a big, big blow for us.
"But it's football, it's happened, and my duty is to find a solution, to be competitive like we have been for many years."
It had already been a frustrating start to the season for Rodri, missing City's first four games including the Community Shield with a hamstring injury.
The Arsenal clash had been his first start and he was forced off after twisting his knee and going down following minor contact with Thomas Partey.
Rodri joined City in 2019 from Atletico Madrid and has made 260 appearances in all competitions, helping the club to four titles as well as the Champions League – in which he memorably scored the winning goal in the final – in 2023.
