text: if jackie robison's number (42) was retired, then why does mariano rivera have that number?   his uniform number is 42, which has been retired by all major league baseball teams since 1997 in honor of jackie robinson. however, rivera is permitted to use the number due to a grandfather clause, and he is the last active major league player to wear the number. given the many uniform numbers retired by the yankees, it is very likely his number would have been retired by them after his career ended anyway.