[3][4] He made his professional debut with the Low–A Lowell Spinners, logging a 4.50 ERA in 12 starts.

Zeferjahn did not play in a game in 2020 due to the cancellation of the minor league season because of the COVID-19 pandemic.

[6] Zeferjahn split the 2022 campaign between the High–A Greenville Drive and Double–A Portland Sea Dogs.

[8] Zeferjahn began the 2024 season with Portland, and was promoted to the Triple–A Worcester Red Sox after seven scoreless appearances.

On August 23, Zeferjahn was selected to the 40-man roster and promoted to the major leagues for the first time.