The best one I can think of is Eddie Guerrero. When you bring up "biggest stars of the 20th century", he's not one of the guys you'd immediately say, but then when someone brings him up you wonder why on earth you'd leave a guy like that out.
He's a nice rags-to-riches story, being cut immediately and famously getting a job as a guy bagging groceries and giving up on an NFL career before making 3 super bowls (the only 3 years he started for an entire season) and setting records in each one. When he finally got a chance he made it big, but he never reached the popularity of guys like Manning and Brady because he was injury-prone and played most of his career in St. Louis and Arizona, two jobber franchises that have sucked throughout their entire existence except when he was there.
Maybe that'll help you guys figure out an equivalent.