Bello started 2021 with Greenville before being promoted to the Double-APortland Sea Dogs.[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.[8] Bello made six starts for Greenville and 15 starts for Portland, going a combined 7–3 with 3.87 ERA while striking out 132 batters in 95+1⁄3 innings.[2] On November 19, in advance of the Rule 5 draft, the Red Sox added Bello to their 40-man roster.[9]
Bello began the 2022 season with Portland.[10] He was ranked 97th in the list of baseball's top 100 prospects by Baseball America.[11] On May 5, Bello threw a seven-inning no-hitter in the second game of a doubleheader against the Reading Fightin Phils; it was the fifth no-hitter in Sea Dog history.[12] Bello was promoted to the Triple-AWorcester Red Sox on May 16.[13] In his first nine games with Worcester, he posted a 6–2 record and a 2.81 ERA while striking out 72 batters in 51+1⁄3 innings.[14]
On July 4, the Red Sox announced that Bello would be promoted to the major leagues for the first time.[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.[16] He made another start on July 11, then was optioned back to Worcester.[17] Bello was selected to the 2022 All-Star Futures Game but replaced on the roster due to his major-league promotion.[18] He was recalled by Boston on July 22.[19] On August 4, he was placed on the injured list with a left groin strain.[20] He rejoined the team on August 24.[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.[23] In 18 minor-league games (17 starts) he went 10–4 with a 2.34 ERA while striking out 129 batters in 96 innings.[2] Bello was named the minor-league Starting Pitcher of the Year by the Red Sox organization.[24]
Bello began the 2023 season on the injured list with right elbow inflammation.[25] He was activated on April 17.[26] After posting a 9.82 ERA in two starts, he was optioned to Worcester on April 24,[27] then recalled on April 28 when Garrett Whitlock was placed on the injured list.[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.[23]
On March 9, 2024, Bello signed a six-year, $55 million contract extension with the Red Sox including a seventh-year club option for $21 million.[29] During spring training, he was named the team's starter for Opening Day of the 2024 season.[30] Due to lat muscle tightness, Bello was on the injured list from late April until being activated on May 12.[31]
