Right-hander Vin Mazzaro was optioned to Triple-A after taking the loss against the Blue Jays on Tuesday night, allowing six runs and 13 hits in a 6-3 defeat.
A six-year veteran, Maier was batting .172 with two home runs and seven RBIs. Kansas City's first-round draft pick in 2003, he and infielder Irving Falu were the longest tenured players on the active roster. Seldom used this season, Maier had just nine at-bats since May 27.
Adcock joins the Royals for the fifth time this season. He is 0-3 with a 3.12 ERA in nine games, including two starts.
Coleman has made 17 appearances for the Royals over three separate stints in the majors this season. He's 0-0 with a 4.91 ERA in that time.