Chelsea goalkeeper Thibaut Courtois was absent for training with the Premier League club on Monday.
This comes amid his link with a move to Real Madrid.
It is reported that the Blues do not know when the 26-year-old Belgium international will return.
Courtois has been a Chelsea player since 2011, when he signed from Belgian club Genk and was loaned to Atletico Madrid.
He has played 154 times for Chelsea, keeping 58 clean sheets and conceding 152 goals.
Chelsea winger Eden Hazard has also been linked with Real, but did attend training on Monday.