[2][3] He made his professional debut in 2014 with the Dominican Summer League Red Sox, appearing in 14 games while compiling a record of 1–1 with a 2.89 ERA.

In 2018, Hernández pitched for the High-A Salem Red Sox and Double-A Portland Sea Dogs,[4] appearing in a total of 28 games while compiling a 9–5 record with 3.35 ERA.

[6] In 2019, Hernández had a 0.82 ERA with the Red Sox in spring training, second only to Chris Sale, and was optioned to Double-A Portland prior to Opening Day.

[7] On April 23, Hernández was added to Boston's major league active roster for the first time, as the 26th man for the second game of a doubleheader.

[8] He made his MLB debut that day,[9] pitching 2+1⁄3 scoreless innings against the Detroit Tigers while striking out four; he was optioned back to Portland following the game.

[12] In that start, Hernández struck out the first four batters he faced, but issued five walks in three innings, while allowing four runs on three hits and taking the loss.

[22] Overall during the regular season, Hernández made 48 appearances with Boston, all in relief, compiling a 2–2 record with 3.38 ERA; he struck out 54 batters in 40 innings.

[20] Hernández was designated for assignment on January 6, 2023, to make room on the team's 40-man roster due to the signing of Justin Turner.