Evangelista "Cyborg" Santos told MMA Fighting that he suffered a fractured skull during his loss to Michael Page at Saturday's Bellator 158 in London.
In the second round of their welterweight bout at the O2 Arena, Santos was dropped by Page with a flying knee to the head.
"I'm under observation now and will have surgery in the United States," Santos told MMA Fighting. "I had a serious injury, but Bellator is giving me all the support I need."
His ex-wife, MMA fighter Cris "Cyborg" Justino, took to Instagram to ask for thoughts and prayers.