There was no Mercury astronaut assigned to fly on Apollo 13.

The Apollo 13 mission had a crew of three astronauts:

* Jim Lovell (Commander)

* Jack Swigert (Command Module Pilot)

* Fred Haise (Lunar Module Pilot)

These astronauts were all part of the Apollo program, not the Mercury program. The Mercury program was a much earlier program that focused on single-person missions to Earth orbit. The Apollo program was focused on lunar missions.