Cassel threw 16 of 25 for 248 yards and two touchdowns as the Vikings beat the Steelers 34-27 in London as part of the NFL international series.
However, Cassel was only playing due to an injury to regular quarterback Christian Ponder, and Frazier suggests that there would not be a change in their number one choice despite his performance.
"He did a good job from what I can tell. It's been a while since I've been able to stand up here and not talk about us turning the ball over. So we can be a good football team when we don't turn the ball over, we know that. Matt did a terrific job in that area. Made some nice throws, so very, very happy for him and what he was able to get accomplished. He did a great job of engineering our team to a win," Frazier told reporters.
"I don't think there will be much discussion about [changing quarterback]. We'll be talking about our team, what we've got to do, and how we're going to improve our football team going forward.
"If you're asking, our quarterback is Christian Ponder. Now we'll discuss a lot of things when we come out of the bye, but we'll talk about a lot of things. We're going to enjoy this victory for a little while at least."
The win is the Vikings' first of the current NFL season, taking them to 1-3 while the Steelers drop to 0-4.