Saunders, who will be working on normal rest, is 3-7 with a 6.48 ERA in 11 career starts against the Rangers, and 0-6 with a 9.38 ERA in Texas, with 41 hits allowed, 16 walks and 13 home runs in 31 2/3 innings.
Showalter isn't certain that rookie Steve Johnson's left knee is completely healthy. He's also comfortable playing the veteran hand.
"Joe's rested, ready to go," Showalter said. "Obviously, experience. He's pitched some in the playoffs and we had a couple options. Not 100 percent sure about Steve's knee, and things went well yesterday.
"We've said many times this year, our No. 1 starter is the guy that's pitching that day. Our whole rotation has been the sum of the parts. Joe's pitched well for us since he's been here. And he's got a tough task ahead of him."
Showalter isn't concerned about Saunders' past struggles against the Rangers, and particularly in their home ballpark.
"You can make numbers look any way you want to," Showalter said. "If you're looking for pitchers with good numbers against the Texas Rangers, they're hard to find. Regardless of who pitches tomorrow, we're going to have to pitch well and do some things out of the bullpen and swing the bat and catch the ball. It's not one phase of the game, but it is very important.
"It's an opportunity for Joe. He's pitched some good games, too. If you look at pure numbers, which I have, you look at this year's stuff and previous years, guys change and players change."
Saunders claimed that he couldn't recite his career statistics here.
"I can imagine, but I don't care," he said. "Tomorrow is a new day."
Showalter will have a deep bullpen after his roster is set - the deadline is 10 a.m. on Friday - and can back up Saunders if the left-hander falters again.
Johnson had a legitimate chance without the health factor.
"It's part of it," Showalter said. "It also increases our opportunities if we're able to continue to play, with the people we have to pick from. It's good that we have people to pick from. And there were some factors there that leaned toward Joe."
Showalter didn't want to use Jason Hammel before Saturday after he threw 73 pitches in Monday's simulated game in Sarasota.
Update: X-rays taken on Mark Reynolds' foot came back negative.
He fouled a ball off it in last night's game and was in obvious pain, but is expected to be in the lineup Friday.