Liverpool winger Raheem Sterling did not report for training on Wednesday citing illness, sources told ESPN FC.
The development comes after Sterling, 20, requested not to participate in the club's preseason tour of the Far East and Australia.
Sterling is now set to be assessed by a club doctor, as is standard practice at Liverpool when there are doubts regarding a player's health.
Neither the club nor the player has made any comment regarding his absence.
Sterling, 20, has been the subject of two failed bids from Manchester City this summer, the second of which was £40 million. The player, who has two years remaining on his contract at Anfield, rejected an offer of a new £100,000-per-week deal earlier this year.
Last month, ESPN FC sources said City's hierarchy have given the green light for the club to make a £50 million offer for the England international.
Brendan Rodgers' Liverpool squad are due to fly out on Sunday ahead of their first friendly against a Thai All-Stars side in Bangkok next Tuesday.
Sources have told ESPN FC that Sterling is unhappy at how he was asked to play out of position on occasion last season.
The winger was used in a right-back role in some games, including the defeat to Manchester United in March, despite impressing in a forward role from December onwards because of Liverpool's lack of striking options.
Former Liverpool defender Jamie Carragher believes the way the whole affair is playing out could be causing long-lasting damage to Sterling's standing.
"He's starting to get a reputation that could be hard to rid himself of in the future," Carragher wrote on www.kicca.com. "Are Man City fans looking at the situation and thinking this could be us in a few years?
"I think this situation will only make Liverpool more determined to get the fee they feel Raheem is worth rather than caving in to Raheem's and Man City's wishes."
ESPN FC correspondents Mike Whalley and Peter O'Rourke contributed to this report