The Legion said in a statement late Monday that the Rev. Alvaro Corcuera was undergoing tests in Hartford, Connecticut, to determine a course of treatment.
Corcuera unexpectedly turned over the day-to-day running of the Mexican-based Legion in October, saying he didn't have the energy to oversee the radical reform of the congregation that was ordered by the Vatican.
The Legion has been in turmoil ever since it acknowledged in 2009 that its founder, the late Rev. Marcial Maciel, had sexually abused his seminarians and fathered three children.
The Vatican took it over in 2010 after determining the Legion had been infected by Maciel's influence and needed to be "purified."