The midfielder has been plagued by physical problems in the past few seasons, and his club fear he could now miss another two months of action
The Netherlands international has been out of action ever since he picked up a hamstring problem back in November while away on international duty, but he recently resumed regular training action again.
However, the Barcelona loanee, 26, has endured a setback at Schalke's training camp in Dubai, and the club have announced on their official website that Afellay faces yet another spell on the sidelines.
It is not yet known how long the attacking midfielder will be out of action, but Schalke fear that they will be without Afellay for up to two months.
He joined the club on a one-year loan deal at the start of the season and has since made 15 official appearances for the Konigsblauen, scoring four goals in the process.