Matt Cassel knows that Teddy Bridgewater is waiting in the wings. He knows that the Vikings didn't draft the kid in the first round to hold a clipboard forever. But as Cassel prepares to start the season opener against the St. Louis Rams, Cassel says he can't play well while looking over his shoulder.

"You can't play that way," the 10-year veteran said today. "You can't worry that if I make a mistake, what's going to happen next? You've got to go out and play the game to the best of your ability. I think Teddy does have a tremendous future, but hopefully, as long as I'm here, he's not playing."

Cassel didn't falter with Bridgewater nipping at his heels early in training camp. He played well in the first two preseason games to pull away from Bridgewater, who struggled a bit in the preseason opener. Cassel completed 66.7 percent of his preseason passes for 367 yards and two touchdowns. He averaged 9.4 yards per attempt — nearly four yards better than what Bridgewater averaged.

Head coach Mike Zimmer knows he must walk a tightrope at quarterback. He wishes he could hold Cassel and Bridgewater to the same standard as he plans to hold the other positions, and go with whoever is playing best at that point in time. But he acknowledged that the ripple effect is more significant when changing quarterbacks than when changing out starters at other positions.

"That's something you have to consider," Zimmer said. "With every position, even with the middle linebacker or the safety, you don't want to make, as you say, a knee-jerk decision just because they don't play well one day or do the things you think they're capable of. I think you still have to go on what you see every day in practice and what you see in the preseason."

That's why Zimmer said he doesn't foresee the Vikings making a change early in the season.

"I know the fans are excited about Teddy. I'm excited about Teddy," Zimmer said. "But we have to do what's best for the football team at this point in time and what's best for the future. … As you know, coaches' futures aren't necessarily very long. I'm always trying to try to think between the now and the future and what's best for this organization as we go down the road."