Russia star Alan Dzagoev requires a scan on a hamstring injury that could rule him out of the rest of the World Cup.
Playmaker Dzagoev went down midway through the first half of Russia's tournament opener against Saudi Arabia on Friday (AEST).
"Alan Dzagoev damaged the muscles in the back of the thigh," Russia said in a brief statement.
"The terms of treatment will be determined by the results of the upcoming MRI [scan]."
Despite Dzagoev's injury, Russia led 2-0 at the break in Moscow thanks to goals from Yuri Gazinskiy and substitute Denis Cheryshev.