Miller was 2-0 with a 3.63 ERA in five spring starts, giving up seven earned runs on 22 hits and four walks over 17 1/3 innings. Prior will make his third start and fourth appearance of Spring Training on Wednesday. So far, Prior has a 9.82 ERA, giving up eight runs on 12 hits and seven walks over 7 1/3 innings.
Cubs manager Lou Piniella confirmed Miller won the job before Tuesday's game.
"He's going to be our fifth starter," Piniella said of Miller, who spent last season rehabbing his right shoulder. The right-hander did make five starts in September. "He's pitched well in camp and has earned a spot.
"Remember our posture here with both Prior and [Kerry] Wood has been the same," Piniella said. "It hasn't changed since we got here. Whenever they're ready, they're ready. I've seen some nice improvements from Mark. He's worked hard. The other kid [Miller] has pitched well. In fairness, what do you do?"
Prior will make one more Cactus League start on Wednesday against the Colorado Rockies. What the Cubs do with him remains unclear. He could open on the disabled list for the fourth straight year and stay in Arizona for extended Spring Training, or he could be assigned to Triple-A Iowa.
Piniella didn't know what will become of Prior, an 18-game winner in 2003 who has been on the disabled list at least once every season of his big-league career. Prior was to be paid $3.575 million this year, and his contract included additional money linked to performance bonuses.
"We'll figure that out in the next couple days," Piniella said. "Obviously, he needs to continue pitching. I don't have an answer for that."
This will be the first time Miller opens the season on the big-league roster since 2004. The 29-year-old has a 62-45 record and a 4.00 ERA in his career.
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.