INDIANAPOLIS -- The Indianapolis Colts have no choice but to start Jacoby Brissett against the Arizona Cardinals on Sunday if they expect to have a chance to at least tread water until Andrew Luck returns. Scott Tolzien showed against the Los Angeles Rams that he's not capable of having command of the offense.

What the decision to switch to Brissett also means is that the Colts are acknowledging that they made a mistake in not acquiring another quarterback to challenge Tolzien during the offseason. General manager Chris Ballard constantly talked about adding competition on the roster, but he firmly put his foot down and held out on addressing the quarterback position during offseason workouts and the June minicamp.

But the uncertainty with Tolzien started to show when the Colts unloaded 2015 first-round pick Phillip Dorsett to the New England Patriots for Brissett. Colts coach Chuck Pagano said he's not ready to announce who will start against the Cardinals, but ESPN's Adam Schefter reported Friday that the decision had been made and the team was going with Brissett.

The Colts felt confident -- publicly at least -- that Tolzien's experience in the offense would be enough to maintain the job while Luck rehabbed from January shoulder surgery. Tolzien was with the Colts last season and even started a game while Luck was out with a concussion. And he ran the first-team offense from April up until their Week 1 loss.

The signs were there during training camp indicating Tolzien might have issues. He struggled throwing the deep ball and with accuracy against the Detroit Lions in the joint practice between the teams in early August. And the Colts lacked any offensive flow with Tolzien during the preseason. He didn't lead them to a touchdown until the third game and Pagano started him in the finale, which is often meant for bottom-of-the-roster players trying to make a final push.

Tolzien sealed his fate by throwing two pick-sixes and not being effective against the Rams in Week 1. He was 9-of-18 for 128 yards to go with four sacks and the two interceptions.

The challenge now for the Colts and Brissett is to come up with a game plan simple enough that will allow him to be effective because he has had less than two weeks of practice with his new team.

Brissett, who started two games with the Patriots last season, has better arm strength and is more mobile than Tolzien. He should also energize an offense that gained only 225 yards against the Rams.

"He's a big guy," offensive coordinator Rob Chudzinski said. "Has a real calmness in the pocket so he's able to move around in the pocket and evade some of the rush. He gets out. He scrambles well. He's got a strong arm. He's been able to produce and score and lead drives and all those types of things. As you get to know him a little more, he's a smart, smart football player and picks things up well. He's got a mental toughness about him and just a presence about him that I really like."