Cassel injured his foot on a scramble during the Vikings' field-goal drive early in the second quarter. He was carted to the locker room for X-rays after trainer Eric Sugarman examined his foot. It was later confirmed by ESPN.com that Cassel suffered several fractured bones in the foot.
Cassel was seen on crutches in the locker room following the team's 20-9 loss to New Orleans.
Bridgewater completed 12 of 20 passes for 150 yards in relief of Cassel and completed his first NFL pass to Greg Jennings for 10 yards.
The Vikings named Cassel their starting quarterback at the beginning of the season, and had planned not to have a "quick hook" with him, as coach Mike Zimmer put it.
Bridgewater's sudden starter status is the latest twist in a season that's already deviated significantly from the Vikings' plans, given the off-field trouble that forced the team to eventually place star running back Adrian Peterson on the commissioner's exempt list.