Son Heung-min was forced off with a worrying-looking head injury in Tottenham's Champions League match with Marseille.
A 23rd-minute aerial collision between Son and Marseille defender Chancel Mbemba led to a lengthy break in play.
In visible pain, Son was eventually helped off the field by Spurs' medical team as he struggled to walk.
The incident will spark fears for both Tottenham and South Korea, with Son's national team due to open their World Cup campaign against Uruguay in just 23 days.
Whether South Korea's star man will be fit for that match on November 24 remains to be seen, as they look to make it out of the World Cup group stage for the first time since the 2010 edition in South Africa.
Spurs will be in action three more times before the World Cup starts, with Premier League fixtures against Liverpool and Leeds United either side of a third-round EFL Cup meeting with Nottingham Forest.