The Boston Red Sox players are wearing number 21 as part of Major League Baseball's tradition to honor Roberto Clemente, a Puerto Rican Hall of Famer who wore number 21 throughout his career. On Roberto Clemente Day, which is recognized annually in mid-September, MLB allows certain players, especially those of Puerto Rican descent, Roberto Clemente Award nominees and recipients, to wear number 21 to celebrate his legacy and contributions to baseball and society. This is a tribute to Clemente's achievements on and off the field, including his status as the first Latin American player to reach 3,000 hits and his humanitarian work.

This practice extends across the league with players and personnel wearing patches or the number 21 on their jerseys to commemorate Clemente and his impact, particularly around September 15, which is set as Roberto Clemente Day in perpetuity. The Red Sox and other teams participate in this honor as part of broader celebrations during Hispanic Heritage Month.

Hence, the Red Sox wearing number 21 is part of this league-wide tribute to Roberto Clemente.