The
Boston Red Sox chose Casas in the first round, with the 26th overall selection, of the
2018 MLB draft. He signed with the Red Sox, receiving a $2,552,800
signing bonus, the full slot value for the pick, and a salary of $774,000 per year. He was assigned to Boston's
Rookie League team, the
Gulf Coast League Red Sox. In a game on June 25, Casas was injured while playing third base; he subsequently underwent season-ending surgery on June 29, to repair a torn
ulnar collateral ligament in his right thumb. Casas began 2019 with the
Greenville Drive of the
Single–A South Atlantic League. While playing for the Greenville Drive, Casas was named the South Atlantic League Mid-Season All Star in 2019. In early June, he was named to the South Atlantic League All-Star Game. In mid-June, Casas was added to the top 100 prospects list of
Baseball America, at number 98. In late August, he was named a South Atlantic League Postseason All-Star, and recognition from
Baseball America as the Red Sox 2019 Minor League Player Of The Year. On September 1, Casas was promoted to the
High–A Salem Red Sox. In mid-September, he was named the Red Sox' minor league offensive player of the year. Over 122 games between the two clubs, Casas
slashed .254/.349/.476 with 20 home runs and 81 RBI. During 2020, with no minor league season due to the
COVID-19 pandemic shutdown, the Red Sox added Casas to their pool of reserve players on August 20, so he could participate in intra-squad workouts. He was subsequently invited to participate in the Red Sox' fall instructional league. Following the 2020 season, Casas was ranked by
Baseball America as the Red Sox' number one prospect. Casas began the 2021 season in
Double-A with the
Portland Sea Dogs. In addition to playing 77 games for Portland, Casas also appeared in nine games for the
Triple-A Worcester Red Sox, batting a combined .279 with 14 home runs and 59 RBI. and was named the starting first baseman for the East team in the league's annual
Fall Stars Game. The Red Sox invited Casas to
spring training as a non-roster player in 2022. He returned to Worcester to start the season. In May 2022, he was ranked 18th in the list of baseball's top 100 prospects by
Baseball America. Casas sustained a
high ankle sprain in mid-May, causing him to miss over a month of playing time. The Red Sox promoted Casas to the major leagues on September 4, and he made his MLB debut that day. In his debut, Casas recorded his first career hit, a single off of
John King of the
Texas Rangers. Two days later, he hit his first career home run off of
J. T. Chargois of the
Tampa Bay Rays. In 27 games with Boston, he batted .197 with five home runs and 12 RBI. In 72 games with Triple-A Worcester, he batted .273 with 11 home runs and 38 RBI. In January 2023, Casas was ranked 29th in the
Baseball America list of top 100 prospects. Casas was Boston's opening day first baseman to start the 2023 season. However, he struggled in the first month of his first full major league season, hitting only .137 over 24 games in April. However, once April ended, Casas turned it around and established himself as the Red Sox' regular first baseman. For the month of July, he received the
AL Rookie of the Month Award after batting .348 with seven home runs and 13 RBI in 21 games. In mid-August, he missed two games at
Yankee Stadium due to a tooth infection. Casas was placed on the
injured list on September 16 due to right shoulder inflammation. Two days later, Casas was shut down by the Red Sox, ending his rookie season. He batted .263 with 24 home runs and 65 RBI over 132 games. He was subsequently named a finalist for a
Silver Slugger Award, and for the
American League Rookie of the Year Award. Casas began the 2024 season as Boston's regular first baseman. Through 22 games, he had six home runs and 10 RBI, then was placed on the injured list on April 21 due to a rib injury. A fractured rib was subsequently diagnosed, with no clear timetable for his return to the team. On April 27, he was transferred to the 60-day injured list. Casas was activated on August 16. In 2025, Casas got off to a tough start at the plate, slashing .182/.277/.303 with three home runs and 11 RBI in 112 plate appearances. Casas ruptured the
patellar tendon in his left knee running to first base on May 2 and was subsequently placed on the injured list on May 3. He was transferred to the 60-day injured list on May 24. Casas missed the remainder of the season as a result of the injury. Former Red Sox pitcher
Garrett Richards served as a mentor throughout the early stages of Casas' rehabilitation process. Casas plans to return to the roster for the
2026 MLB Opening Day game. ==International career==