Manchester City midfielder Kevin De Bruyne is facing a spell on the sidelines after injuring his right knee in training on Wednesday.
A club statement said the full extent of the injury is not yet known, but the 27-year-old Belgium international was undergoing tests.
De Bruyne, who was later seen on crutches, is expected to fly to Barcelona for further tests with Dr. Ramon Cugat, a specialist who treated him when he suffered a knee injury in January 2016.
Cugat also helped City teammates Ilkay Gundogan and Benjamin Mendy recover from cruciate ligament injuries.
"Kevin is a great player and it's just unfortunate that it's happened to him in training but there's more than enough depth in the squad," City right-back Kyle Walker said at the premiere for the club's upcoming documentary "All Or Nothing."
"We don't just rely on one player. It's a team game and whoever steps in will do well. Obviously, it's a big loss but we've got more than enough cover."
De Bruyne had just returned to training Aug. 6 after helping Belgium to the semifinals of the World Cup in Russia.
He was a second-half substitute in the 2-0 Premier League-opening victory over Arsenal on Sunday, coming on in the 60th minute.
His loss will be a major blow to Pep Guardiola after helping City to a record-breaking 100 points last season on the way to the title. He claimed the most assists for a second successive year although he was beaten by Liverpool's Mohamed Salah for the Player of the Year award.
City do, however, have David Silva, Bernardo Silva and Gundogan who can offset his loss in midfield. And De Bruyne's injury could present an early opportunity for exciting youngster Phil Foden.