The ball is there to be played and it appears that Rubezic does get his head to it first. From this angle I don’t see any foul committed. Hopefully Kyogo is well and there are no long term effects.

This is a situation where the game will need to change to protect players. Both could have been badly injured here, and the current remedy is the faint hope that a player in Rubezic’s position will see the danger to himself and his opponent, acknowledge the risk, and pull out of the challenge. Of course most professional players will never cede a free header like that.