From what I've researched, no one wore number 21 for the San Diego Chargers in 1998.
Ryan Leaf.
George Roberts
RB Glen Bonner.Bonner wore #21 for the Chargers in 1974 and 1975.
Chad Rinehart is number 78 on the San Diego Chargers.
Ryan Matthews
Jeromey Clary is number 66 on the San Diego Chargers.
Malcom Floyd is number 80 on the San Diego Chargers.
Lawrence Guy is number 71 on the San Diego Chargers.
Jonas Mouton is number 57 on the San Diego Chargers.
Kenny Wiggins is number 72 on the San Diego Chargers.
John Phillips is number 83 on the San Diego Chargers.