According to fan repots report from the show, the injury took place when Wyatt was delivering multiple headbutts to Cena in the corner of the ring. Whatever happened that caused the injury was serious enough that Cena’s eye was reportedly swollen shut. There has been some talk that Cena may have suffered a possible concussion as well.
As soon as the injury happened, the referee threw up the “X” symbol and trainers rushed to the ring to check on Cena. To Cena’s credit, after being checked out for a bit, he did finish the match. Here’s is a photo of Cena being attended to by the ringside medical staff: