Argentina and Chelsea goalkeeper Emiliano Martinez was substituted after suffering an injury during the 66th minute of the Premier League meeting with Everton.
Everton had been awarded a free-kick, which was taken by Kieran Dewsbury-Hall. Martinez and Everton defender Jarrad Branthwaite both challenged for the ball and collided heavily head-first.
Both players fell to the pitch, but Branthwaite was able to get back to his feet after a short time. Martinez remained still as medical staff attended to him for several minutes.
The goalkeeper was assessed carefully by the medics before it was decided he could not continue in the away match, part of the eighth round of the Premier League.
Belgium goalkeeper Mike Penders came on in his place. The appearance was the 21-year-old’s first in the English top flight.