Logan Thomas struggles early in first practice as Cards starter

TEMPE, Ariz. -- Rookie Logan Thomas has gone from standing in for San Francisco quarterback Colin Kaepernick on a commercial shoot during training camp to impersonating him in practice in Week 3 to facing him as the Cardinals' starter Sunday afternoon.

If only Thomas' on-field progress went as quickly as his escalation up the depth chart.

Inheriting an offense 17 weeks into a season is easier for Thomas to talk about than to execute. As the third-string or back-up quarterback all season, he hasn't had many opportunities since training camp to learn the intricacies of leading Arizona's offense.

"He's worked, fundamentally, extremely hard and gotten much better,” Cardinals coach Bruce Arians said. "But putting those fundamentals to use in the offense, that's two totally different things.”

Thomas' first practice as Arizona's starter on Wednesday didn't start so well.

Arians said Thomas was "shaky” at first but then finished "pretty strong” as he developed a better understanding of where to go with his passes. Arians wants to see his fourth-round pick control the offense, understand why and where he's throwing it, and to make sure he does it accurately.

Most importantly, however, he wants to see Thomas go through his progressions to find the best option. That's not even the toughest part for Thomas.

"I think it's just this offense as whole, with the hots and sights, and then with the reads,” he said. "But the guys around me have done a great job of making me comfortable.”

Arians isn't the type of coach to talk Thomas through his nerves. It's a process that Thomas had to work through with the help of quarterbacks coach Freddie Kitchens, who talked to Thomas after every series Wednesday.

"It wasn't the greatest, but as it went on, it got better and better,” Thomas said. "It just comes with reps and comes with time.”

Time has become Thomas' toughest opponent this week. He hasn't had much of it in the offense since training camp and the preseason. While that experience will be a good resource for him, it's been four months since he prepared as the starting quarterback, as he did heading into the final preseason game.

The Cardinals fourth starting quarterback this season understands Sunday in Levi's Stadium won't be like his preseason starts.

"I think it's different just because it means something a little bit more, or should I say a lot of bit more,” Thomas said. "It's Week 17 of the regular season. It's December football. Those are obviously huge games.”

This week, he has just three days to absorb the game plan on the practice field. That means Thomas has three days to calm his nerves and get comfortable. He's not letting Arians' promise to have a quick hook if Thomas isn't playing well change how he prepares, but Thomas appreciates Arians' honesty.

"If I prepare the right way, play the way I think I can play and the way I plan to play, then it shouldn't be a problem,” Thomas said.

After saying Tuesday night on SiriusXM NFL Radio that Thomas will play at least the first half Sunday, Arians said Wednesday that there are "no plans on how long” Thomas plays.

The only part of Sunday that won't be new for Thomas will be the novelty of playing. He's seen action in two games during the regular season, having thrown nine passes, completing one. But Thomas feels a week's worth of reps will calm him down and have him ready for his first start.

"Going and doing it is completely different than just seeing it," Thomas said. "So, it's going to be great."