Thomas appeared in two games for Arizona last year, going 1 for 9 for 81 yards and a touchdown. Harnish has been a backup for the first three seasons of his career with the Colts and Vikings and has not appeared in a game.
"He's going to get a ton of work and those reps are invaluable," Arians said. "Just in decision-making, how fast you're getting it out of your hand, where you're going with it, where you're going when 'one' and 'two' are still covered. Are you still scrambling around or are you going to find your outlets? That growth process comes strictly from reps."
Arians still has a plan for the second-year pro.
"We drafted him for two years from now, not November," Arians said. "It just so happened his number got called (earlier.) I was not going to let him fail (in San Francisco) because once you fail those scars never go away.
"It's very hard to get the guys back in the locker room if you cost them the playoffs. I just wasn't going to put him in that situation. Right or wrong, it just was my decision."
Arians earlier said he'd play Thomas for at least the first half. But that was before Thomas failed to inspire confidence at Wednesday's practice.
Thomas has appeared in two games, completing 1 of 9 throws. Starter
Arizona remains a 6-point underdog Sunday.
After earlier saying
Thomas has appeared in two games, completing 1 of 9 passes. That completion was an 81-yard touchdown to running back Andre Ellington.
Quarterback Drew Stanton (knee) isn't expected to be ready until the playoffs, so Lindley is the only option behind Thomas. Lindley has zero touchdowns and eight interceptions in 225 career pass attempts. Arizona has not scored a touchdown in its past two games.
A 6-6, 250-pounder out of Virginia Tech, Thomas had a "shaky" start to Wednesday's practice before finishing "pretty strong," according to Arians.
Arizona is a 6-point underdog Sunday. The line opened at 4.5 before bettors pounced on the 49ers.
Despite their offensive struggles, the Cardinals can win the NFC West with a victory and a Seattle loss to the Rams.
"He's worked, fundamentally, extremely hard and gotten much better," Arians said. "But putting those fundamentals to use in the offense, that's two totally different things."
Thomas has just three days of practice before facing the 49ers in a game that has the possibility of landing his team with a first-round bye with a win and help from other results.
"It wasn't the greatest, but as it went on, it got better and better," Thomas added. "It just comes with reps and comes with time."
The Cardinals are a 6-point underdog in San Francisco Sunday.
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