That article is the first one that i've read that really explained what happened in detail.
He was complaining about "flashers" in his eye the night before the Bears game. During the game, the wind blew his laminated playcard into his eye, scratching it. They say he was lucky the playcard hit him in the eye, because it kind of forced him to get it looked at.
His issue started before the "scratch," and it gave team doctors a bit of an in to check it out. Sugarman scheduled him an 8 AM appointment. Zim gave in but was still reluctant, saying he'd be outta there "by 8:30." Next thing he knows, he's seeing a retina specialist.