Alexander Isak was substituted in the 61st minute of the League Cup second semi-final between Newcastle and Southampton.
Just over 20 minutes later, the Swede received a rock-hard free kick in the head and fell to the ground.
Isak tried to play on but in extra time he was forced to break and was replaced after continuing problems after the blow to the head.
Newcastle won the semi-final 2–1 and progressed to the League Cup final 3–1 on aggregate.
There they face Manchester United or Nottingham, where the former won the first meeting 3–0.
This is the first time in 47 years that Newcastle will play in the final of the League Cup and the final will be played at Wembley on February 26.