The Dodgers are wearing number 21 to honor Roberto Clemente as part of MLB's celebration of Roberto Clemente Day, which occurs every September 15. MLB allows players who are nominees or previous recipients of the Roberto Clemente Award, as well as Puerto Rican players, to wear the number 21 jersey in honor of Clemente. Mookie Betts and others wore No. 21 jerseys because Betts is a nominee for the Roberto Clemente Award, which recognizes outstanding sportsmanship, community involvement, and philanthropy.

Roberto Clemente was a Hall of Famer and humanitarian, celebrated for his contributions both on and off the field, and he tragically died delivering aid to earthquake victims in 1972. MLB commemorates his legacy by allowing special recognition on Roberto Clemente Day with the number 21.

Thus, the Dodgers are wearing 21 not as their regular uniform number but as a tribute to Clemente and to highlight players nominated for the Roberto Clemente Award.