Booser split the 2016 campaign between Cedar Rapids and the High–A Fort Myers Miracle. In 21 appearances out of the bullpen for the two affiliates, he compiled an 0–4 record and 8.53 ERA with 34 strikeouts across 25+1⁄3 innings pitched.[7]
In 2017, Booser was suspended 50 games after testing positive for cannabis.[8][9] After making three appearances for Fort Myers during the season, Booser retired on November 20, 2017.[2] Booser returned to the Seattle area to work as a carpenter.[2][10] Missing baseball, he began to give pitching lessons in 2020, and found that he could throw a fastball at 96 miles per hour (154 km/h).[2]
On February 16, 2022, Booser signed a minor league contract with the Arizona Diamondbacks. He pitched for the Amarillo Sod Poodles of the Double–ATexas League,[12] and was released on July 18. With the Sod Poodles, Booser pitched to a 1–1 record and 6.48 ERA with 30 strikeouts in 19 relief appearances.[5]
On February 6, 2023, Booser signed with the Boston Red Sox and pitched for the Worcester Red Sox of the Triple–AInternational League, logging a 4–3 record and 4.99 ERA with 66 strikeouts across 48 relief outings.[11] In 2024, the Red Sox invited him to spring training as a non-roster player.[10] On April 19, 2024, Booser was selected to the 40-man roster and promoted to the major leagues for the first time.[14] He made his major-league debut that evening, pitching the ninth inning of the Red Sox 8–1 win against the Pittsburgh Pirates.[15][16] Booser was optioned to Triple-A Worcester on June 24[17] and recalled to Boston on July 5 after reliever Chris Martin was placed on the injured list.[18]
Booser finished the 2024 season with a 3.38 ERA and 43 strikeouts in 43 relief appearances for Boston. The Red Sox named him the winner of the Tony Conigliaro Award for the season, which recognizes a "Major Leaguer who has overcome adversity through the attributes of spirit, determination, and courage."[19]
Chicago White Sox
On December 21, 2024, the Red Sox traded traded Booser to the Chicago White Sox in exchange for minor league pitcher Yhoiker Fajardo.[20]