World Cup hosts Russia look set to be without Aleksandr Kokorin after the Zenit striker ruptured cruciate ligaments in his right knee on Thursday.
Kokorin left the field after eight minutes of Zenit's Europa League clash with RB Leipzig, the last-16 second leg ending 1-1 to send the German side through 3-2 on aggregate.
The 26-year-old was seen clutching his right knee as he was substituted and Zenit confirmed on Friday that the diagnosis did not look good.
"Aleksandr Kokorin has been diagnosed with a rupture of the cruciate ligaments in his right knee," a club statement read.
"The Russian international striker will soon head to Rome for treatment. Details of any planned surgery will be announced shortly."
Although no timescale was put on Kokorin's recovery, it appears unlikely he will be ready for this year's World Cup – where he would have been a key member of Stanislav Cherchesov's squad.
Kokorin - who has previously been linked with a move to Arsenal - has scored 12 goals for Russia in 48 appearances and found the net on 19 occasions in all competitions for Zenit this season.