Schalke defender Matija Nastasic has been ruled out for up to 12 weeks with a knee ligament injury, leaving him in a race against time to be fit for the World Cup.
The centre-back was taken off in the second half of last Saturday's 3-2 Bundesliga defeat to Hamburg and tests have identified damage to the posterior cruciate ligament in his right knee.
Schalke expect Nastasic to miss the rest of the season and his chances of being fit in time for Serbia's opening game at Russia 2018 against Costa Rica on June 17 are also in doubt.
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"In last Saturday's clash with HSV, Schalke's centre-back was substituted with a quarter of an hour to play due to a knee problem," the club said in a statement. "The following diagnosis showed the injury in his knee that will rule him out for eight to 12 weeks.
"The 25-year-old will miss the rest of Schalke's season, whether he will feature for Serbia at the World Cup this summer in Russia is still unclear."
Nastasic, who joined on a permanent deal from Manchester City in 2015, has made 24 appearances in the Bundesliga this season.