Leon Goretzka suffered a head injury and required extensive treatment for several minutes during Schalke's Tuesday morning session, before being helped off the field by two supervisors, according to kicker reports.
The 20-year old midfielder has spent lengthy spells on the sidelines this season as he battles against a recurring thigh problem and only returned to full training yesterday.
The club confirmed that there had been some slight bruising to the skull of Goretzka and that he'd miss out the afternoon session for "security reasons."
The German U21 international has managed just six games all season for Schalke, having missed the whole of the hinrunde through injury. Goretzka has managed no more than a single half in the six games.
Goretzka is contracted to the Veltins-Arena club until the summer of 2018 having joined in 2013 from 2. Bundesliga's VfL Bochum for just under €4 million.
Also missing from Tuesday's training were teen sensation Max Meyer, Sead Kolasinac (both flu) and club captain Benedikt Höwedes (ankle).