Finn Balor was recently seemed injured. That's why he wasn't able to compete in WWE Buenos Aires and also pulled off from Mixed Match Challenge.
It was announced during Raw that Apollo Crews will be replacing Finn Balor going forward in the Mixed Match Challenge.
According to PWInsider, Finn Balor is not injured but is ill. Balor is fine to face Drew McIntyre this weekend, which is why he is still being advertised for this Sunday's WWE TLC PPV.
Balor also issued the following apology:
I am sorry I could not preform in #WWEBuenosAires
— Finn Bálor forEVERYone (@FinnBalor) December 7, 2018
Lo siento no pude competir en #WWEBuenosAires