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.
Santos, 38, detailed the injury to MMA Fighting as a serious fracture in his forehead.
Despite the injury, he said he plans to continue his MMA career.
"I'll be back soon," he told MMA Fighting.
Page took to Twitter on Sunday to wish Santos a speedy recovery.
Page had made headlines Saturday when he celebrated the victory by putting on a Pokemon cap and rolling a Pokeball at Santos in an ode to "Pokemon Go," the popular mobile app.