But it wasn't the entire reason.
Saunders has a bruised left hand, the result of getting jammed by a pitch in batting practice prior to his start last Wednesday.
"I think it had a little bit of effect on his last game," Gibson said of Saunder's start against the Dodgers, when he allowed five runs in six innings. "So I just wanted to give him an extra day as well."
Collmenter has dominated the Brewers in two appearances this season. The right-hander did not allow a run in 14 innings against Milwaukee in a pair of starts this year.
The first time that Collmenter faced the Brewers was at Miller Park, when he tossed six innings. He then allowed just three hits over eight innings at Chase Field.
Collmenter, who has a funky over-the-top delivery, last pitched on Sept. 25.
"Collmenter has been good on extra rest this year, has been fairly decent against the Brewers," Gibson said. "A little bit of deception from our first two guys."
Saunders' hand should not prevent him from pitching Game 4, and it did not sound like Gibson would bring Game 1 starter Ian Kennedy back on short rest for Game 4. As it stands now, Kennedy would pitch a Game 5 if necessary.
"I'm not saying I wouldn't do it, but I'd probably steer away from it," Gibson said of using a pitcher on short rest in the playoffs. "We've played as a team all year. I've stayed in the rotation most of the year, have given guys extra time. As I sit here today I'd probably stay away from that. I'm not saying I wouldn't change my mind. I continue to let the guys who have pitched and played all year play, and we'll have Ian for the fifth game if we need that."
Saunders is the lone starter with postseason experience, having started once for the Angels in the 2008 American League Division Series and twice in the 2009 AL Championship Series. The left-hander faced the Brewers on July 20 this year at Chase Field and did not get a decision after allowing two runs over seven innings.