Earlier Monday, manager Eric Wedge had indicated Datz was expected to work the rest of the homestand before sitting out the upcoming road trip to Toronto and Pittsburgh, but that changed shortly before the game.
Stearns, 61, previously served as a scout and, before that, as a Minor League manager with the Blue Jays, Reds, Mets and Nationals organizations.
When Datz revealed on Saturday that he had cancer, he said it was "very possible" he would miss some games, though he said in a statement he is not yet ready to publicly address the issue.
Datz, 53, has served as the Mariners' third-base coach for the past three seasons and has been a member of Wedge's coaching staff since 2003 with the Indians.
Greg Johns is a reporter for MLB.com. Follow him on Twitter @GregJohnsMLB as well as his Mariners Musings blog. This story was not subject to the approval of Major League Baseball or its clubs.