Cassel broke several bones in his left foot in the second quarter of last Sunday's 20-9 defeat at the New Orleans Saints.
He was replaced by rookie Teddy Bridgewater, who will now assume the role of starter for the rest of the campaign.
The Vikings traded back into the first round of the 2014 NFL Draft to select Bridgewater with the 32nd pick after the 21-year-old enjoyed a stellar career in college at the University of Louisville.
Bridgewater completed 12 passes for 150 yards against the Saints and also had six carries for 27 rushing yards, although he was unable to lead the Vikings to a touchdown.
The Miami-born signal-caller will make his first start versus the Atlanta Falcons at TCF Bank Stadium on Sunday.