A former Super Bowl champ says his face was busted open BAD during an NFL game -- opening a gash that needed 17 stitches to close ... and he blames the whole thing on a shoddy helmet.
The guy behind the lawsuit is ex-Chicago Bears safety Ryan Mundy -- who says he was involved in a hit during a 2014 pre-season game against the Seahawks.
Mundy says as a result of the collision, the sharp plastic edge on the padding, lining the inside of his Schutt helmet, sliced into his forehead and opened up a 5 AND A HALF INCH GASH.