A corresponding move will take place prior to Monday night's game against the Mariners.
Guilmet, selected off waivers from the Blue Jays on May 13, spent parts of two seasons in the Major Leagues with the Indians (2013) and Orioles (2014).
The moves are the Rays' latest made in an attempt to keep the bullpen healthy, as they have opted to have eight relievers, which leaves the team with three position players on the bench.
Romero arrived on Saturday, pitched three innings, then was given the news he had been optioned back to Durham.
Rays manager Kevin Cash told reporters on Saturday that he is more comfortable for the time being having the extra arm -- 13 pitchers total -- as opposed to having the extra player on the bench.
Bill Chastain is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.