The Blue Jays will have to make an additional move to create a spot on the 25-man roster but that is not expected to take place until Gillespie officially joins the team.
"He won't be here today, I expect him to be here tomorrow," manager John Gibbons said Saturday afternoon. "Right-handed bat, take a shot and see how he looks."
Gillespie hit .254 with a pair of doubles, a home run and five RBIs in 34 games for the Mariners this season. He has appeared in 112 games over the course of four big league seasons and has a career average of .233 with four homers and 25 RBIs.
The arrival of Gillespie will lead to the departure of either Darin Mastroianni or Brad Glenn. The Blue Jays have been searching for ways to improve their production against left-handed pitching after entering Saturday's game with an AL-worst .234 average vs. southpaws.
Gillespie is the latest in a long line of players who will attempt to fill that void. Even though that's the goal, Gillespie has a very unspectacular .221 average and .557 OPS against lefties in his career. Those numbers are better this season with a .280 average and .677 OPS but that comes in just 50 at-bats.
Gregor Chisholm is a reporter for MLB.com. Read his blog, North of the Border, and follow him on Twitter @gregorMLB. This story was not subject to the approval of Major League Baseball or its clubs.