In 2019, he played in Single–A for the Greenville Drive, compiling a 5–10 win–loss record in 25 games, all starts, with a 5.43 earned run average (ERA) while striking out 119 batters in 117+2⁄3 innings pitched.

[4][5][6] He was selected to the mid-season All-Star Futures Game,[7] and was named the Starting Pitcher of the Year within Boston's farm system at the end of the season.

[15] He made his major-league debut on July 6, starting against the Tampa Bay Rays at Fenway Park, taking the loss after allowing four runs on six hits in four innings.

[21] On September 3, Bello tossed six scoreless innings against the Texas Rangers, striking out five en route to his first major league victory.

[22] Overall with Boston during 2022, Bello pitched to a 2–8 record in 13 games (11 starts) while compiling a 4.71 ERA and striking out 55 batters in 57+1⁄3 innings.

[28] For the 2023 season, Bello appeared in 28 major-league games, all starts, and posted a 12–11 record with a 4.24 ERA while striking out 132 batters in 157 innings.

[30] Due to lat muscle tightness, Bello was on the injured list from late April until being activated on May 12.