Leon Goretzka was taken to hospital after being substituted following a head injury during Schalke's Europa League quarter-final second leg against Ajax.
Goretzka scored the opening goal at Veltins Arena but was forced off in the 84th minute after colliding with Ajax goalkeeper Andre Onana.
The midfielder was assisted while walking off the field and Schalke tweeted shortly after to announce he had been taken to hospital for examination.
"Leon Goretzka is currently en route to the hospital. Still waiting a full diagnosis," the club posted on Twitter.
Guido Burgstaller's goal sent the tie to extra time and Daniel Caligiuri looked to have clinched a semi-final spot for Schalke with their third, but Ajax - who had Joel Veltman sent off in the 80th minute - hit back through Nick Viergever and Amin Younes to advance in dramatic circumstances.
Schalke will continue their challenge to qualify for next season's Europa League when they take on RB Leipzig in the Bundesliga on Sunday.