The veteran quarterback fractured several bones in his left foot during a scramble in the second quarter of a 20-9 loss to the New Orleans Saints on Sunday.
Bridgewater, the first-round pick from Louisville, completed 12 of 20 passes for 150 yards without a touchdown or an interception, leading two drives for field goals.
Cassel was hurt on a 5-yard scramble to set up a field goal early in the second quarter. Coach Mike Zimmer said X-rays revealed the broken bones.
It is not clear how long Cassel will be out, but the injury leaves Bridgewater as Minnesota's quarterback for the foreseeable future.
Bridgewater entered the game on Minnesota's third possession, leading a 10-play, 51-yard drive for a field goal.
Cassel was 5 of 10 for 53 yards before leaving. Bridgewater finished 12 of 20 for 150 yards.
"We can't worry about what we don't have," Zimmer said. "We have to worry about what we do have and how we go forward. No one is going to feel sorry for us. No one is going to cry about we don't have this guy or that guy."
The Vikings' offense now looks completely different than what Minnesota expected as recently as Tuesday. Team officials initially announced star running back Adrian Peterson would play after he missed last Sunday's 30-7 loss to New England.
They later changed course and said he would leave the team indefinitely to deal with child-abuse allegations.
The San Diego Chargers are also short-handed in the offensive backfield. Running back Danny Woodhead hurt his right ankle in a 22-10 win over the Buffalo Bills. Quarterback Philip Rivers said he expects Woodhead to miss at least a few weeks.
Woodhead was hurt on a 1-yard run, when he was stopped by diving defensive tackle Kyle Williams. Williams hit Woodhead directly in the ankle, which appeared to twist underneath the player.
Woodhead was briefly treated on the field before being loaded onto a cart. Starter Ryan Mathews was already out with a sprained right knee.
Baltimore Ravens tight end Dennis Pitta was taken to a hospital with a potentially serious hip injury. Pitta, who played only four games last season after dislocating his right hip in training camp, appeared to re-injure the same hip after catching a short pass from Joe Flacco late in the first half of a 23-21 win over the Cleveland Browns.
The 6-foot-4, 245-pound Pitta took several steps before stumbling and falling without any contact. Pitta stayed on the ground in obvious pain and was consoled by teammates as the medical staff evaluated him.
He was driven to the locker room, then taken to the hospital for further tests.
Pitta's career was in jeopardy before he underwent surgery last year on his hip. He signed a five-year, $32 million contract with Baltimore in March. The Ravens didn't have a postgame update on Pitta.
"He's a good friend, he's a good teammate and he's a hell of a player," Flacco said. "No matter who it is, it's tough to see that happen, especially when it looks like it might be serious."
The most bizarre injury of the day was to Lions linebacker Stephen Tulloch. He left Detroit's 19-7 victory over the Green Bay Packers in the first quarter after hurting his left knee while celebrating a sack of Aaron Rodgers.
Tulloch jumped through the air and looked as if he was trying to mimic the Green Bay quarterback's "title belt" celebration, but then he fell to the ground. He tried to play on, but left the game a short while later.
"I've played nine years in the league and I've never missed a game," Tulloch said. "I've been fortunate enough to take care of my family and do some good things in the league. I made a play on the quarterback to help my team and unfortunately I got up and in the celebration, and it happened.
"I'm not embarrassed."
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