Chelsea goalkeeper Thibaut Courtois, who has been linked with a move to Real Madrid, did not report for training with the Premier League club on Monday.
And it is understood the Blues do not know when the 26-year-old Belgium international will return.
Courtois has been a Blues player since 2011, when he signed from Belgian club Genk and was loaned to Atletico Madrid.
Chelsea winger Eden Hazard has also been linked with Real, but did attend training on Monday, reports BBC.
Courtois was named best goalkeeper at this summer's World Cup in Russia, where Belgium beat England in the third-place play-off.
He has played 154 times for Chelsea, keeping 58 clean sheets and conceding 152 goals.
Speaking on Friday, new Blues manager Maurizio Sarri said he was unsure whether Courtois would stay.
He said: "At the moment Courtois is the goalkeeper of Chelsea. I don't know in the future. It depends on the club, it depends especially on him, but I hope Courtois will be our goalkeeper."
Chelsea were then beaten 2-0 by Manchester City in Sunday's Community Shield with Argentine Willy Caballero in goal.
Afterwards, Sarri said: "I want to hear from Courtois tomorrow (Monday). I want only players with a very high level of motivation."
Chelsea finished fifth in the league last season as they failed to qualify for the Champions League but did beat Manchester United to win the FA Cup.
Following the win over United, Courtois urged the Blues board to invest in the squad.
"We'll see after the World Cup whether I am a Chelsea player for next year," he said at the time.
"You see how Manchester City and Manchester United are spending and working.
"If we want to aim for the top, we should do the same - obviously within the limits of what's possible."
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