With former starter Christian Ponder now cleared to return from a fractured rib, Frazier also said the Vikings haven't decided if Cassel will keep the job beyond this week.
"I think it's always tough when you make a change at quarterback," Frazier said. "You have to take a lot of factors into it when you're doing that. You don't make those decisions lightly and we didn't. We'll see how it plays out. At the start of this ballgame, we'll see how Matt does. But we'll [also] see how it plays out over time."
A permanent switch at quarterback seemed entirely possible after Ponder, who was the 12th overall pick in the 2011 draft, threw five interceptions in the first three games -- all losses -- and Cassel led the Vikings past the Pittsburgh Steelers in London on Sept. 29 for their first win.
Ponder said Frazier informed him of the decision Thursday, and Friday the coach told Cassel.
"It's exciting," Cassel said. "That's why you come out here and you compete through OTAs and minicamp and all that stuff. This week I'll get another opportunity and I'm excited to get back on the field with these guys. Hopefully we can go out there and get another win."
"We don't see [Freeman] being able to play this ballgame yet, based on where we are and where he is with our offense," Frazier said.
Frazier has said Freeman -- who signed a one-year, $2 million deal with the team this week -- will get on the field in the near future.