On December 8, 2021, the Boston Red Sox selected Keller from the Yankees in the minor league phase of the Rule 5 draft.[7] He made 31 appearances (20 starts) for the Triple-A Worcester Red Sox in 2022, compiling a 6-5 record and 3.27 ERA with 126 strikeouts across 113 innings pitched. Keller elected free agency following the season on November 10, 2022.[8]
Hanshin Tigers
On December 20, 2022, Keller signed with the Hanshin Tigers of Nippon Professional Baseball (NPB).[9] He made only three appearances for the farm team in 2023 before returning to the U.S. to have his right elbow examined. On July 13, 2023, without having appeared for the main club, Keller and Hanshin officially parted ways.[10] On August 14, Keller underwent Tommy John surgery, likely ruling him out for game action in 2024.[11]