The Tigers purchased Mercedes' contract after the 2012 season, adding him to their 40-man roster.
The Tigers promoted Mercedes to the major leagues on August 14, 2014. Before being called up, Mercedes had a 4.33 ERA and a 1.33 WHIP in 41 appearances for the Triple-AToledo Mud Hens. Mercedes made his major league debut on August 15, when he pitched two scoreless innings, allowing no hits or walks, and recording two strikeouts. On August 17, Mercedes was optioned back to the Toledo Mud Hens.