According to multiple sources, including PWInsider, Finn Balor suffered a concussion on this past weeks Monday Night Raw during his match with Jinder Mahal.
If you watched the contest, Jinder nailed Balor with a elbow strike to the side of the head. Balor fell to the ground & looked to be briefly knocked out. He may have just been selling extremely well, but the contact on the strike was as legit as you get in a WWE match.
Balor will apparently have to undergo a series of concussion tests before being cleared to return to action.