[8] Simmons was enjoying a successful ride in the Braves bullpen until he landed on the disabled list on July 29 with shoulder soreness.

On August 20, he was temporarily shut down following an unsuccessful rehab assignment with Triple-A Gwinnett, and he didn't return to the mound the rest of the season.

Expected to enter the season competing for the primary setup role to closer Craig Kimbrel, Simmons received unfortunate news when an MRI revealed a tear to his UCL in his throwing elbow, causing him to miss the entire 2015 season.

On February 12, 2015, Simmons underwent successful Tommy John surgery, performed by Dr. James Andrews.

[13] On January 11, 2017, Simmons was traded, along with Mallex Smith, to the Seattle Mariners in exchange for minor-league pitchers Luiz Gohara and Thomas Burrows.