Given there could be an earlier emergency, Rubio's seat was being moved from the MS-22 to a SpaceX Crew Dragon spacecraft, also docked to the ISS, Roscosmos said on Saturday.
"If an emergency evacuation is necessary, Francisco Rubio will return to Earth on it (the Crew Dragon), and the Roscosmos cosmonauts (will return) on the Soyuz MS-22, it said.
"The descent of two cosmonauts instead of three will be safer, as it will help reduce the temperature and humidity in the Soyuz MS-22."
Four other crew members are currently on the orbital station - two more from NASA, a third Russian and a Japanese astronaut, who all arrived in October on the SpaceX Crew Dragon capsule.
Russia has said, however, it plans to quit the ageing project after 2024 and launch its own station.