The club announced that Dana Eveland would start Saturday, and Livan Hernandez would move up a spot to get the start on Friday.

My personal opinion is that Eveland is a fine contestant to start and a change from Petit is good news.

Eveland has gone 1-0 with a 1.65 ERA in Tucson after tearing a tendon in his middle finger.

From Steve Gilbert on MLB.com:

The D-backs have finally settled on a starter for Friday's series opener against the Rockies, and it isn't Yusmeiro Petit or Edgar Gonzalez. And, for that matter, they aren't calling up Dana Eveland to start that night.
Instead, veteran Livan Hernandez will be brought back one day early to start Friday, with Eveland being promoted to start Saturday.

"He's the one guy that if I was going to bring anybody back on three [days], it would be him," manager Bob Melvin said. "He's not a max-effort pitch guy. He's a guy that at times wants to stay out there for 130 or 140 pitches, so he's the one guy that we don't get too terribly concerned about pitches with, and we think he's the one guy that can handle this."

By using Hernandez on Friday, it sets up the rotation so that he will start the final game against the Padres next Wednesday at Chase Field. Hernandez pitched very well against the Padres on Monday, when he allowed just two runs over seven innings.

Hernandez skipped his regular between-starts bullpen session Wednesday and instead threw from flat ground. In addition, Melvin may limit the number of pitches he allows Hernandez to throw Friday. Pitching him on three days' rest again is unlikely.

"No, not on three," Melvin said. "This one just gets us where we need to get with it, and it gets him on San Diego when they come back to our place as well."