DENVER -- Rockies utility man Jeff Baker never received the message "wait your turn" until this season.

Baker was a standout at Clemson, a bonus-baby Draft pick in 2002, and when he wasn't dealing with a series of wrist injuries, a power source in the lineup on Minor League teams.

But with the corners of the infield and outfield occupied for the Rockies, Baker struggled when he did play and has had little opportunity this season. It's different now, with first baseman Todd Helton missing a second straight start Friday night against the Cubs with back spasms. It's Baker's fourth start in 40 games.

That meant Baker received consecutive starts for just the fourth time this season. He can only hope Friday's start is like Thursday's, when he homered in the 10-2 loss.

But Baker, 26, is careful not to see Helton's absence as his audition.

The Rockies kept Baker at Triple-A Colorado Springs almost all of last season, challenging him to prove he could stay healthy, even as he was batting .305 with 30 home runs.

When he was called up in September, he approached it with a relaxed attitude and hit a blistering .368 with five home runs and 21 RBIs in 18 games.

"I'm never trying to prove myself," said Baker, who entered Friday 3-for-8 with the homer and two RBIs for the homestand. "I believe in myself. If you're out there to prove people right or wrong, whatever the reason is, I think you're going to struggle. You're playing the game for the wrong reasons and the wrong motivation."

Baker entered Friday with a .221 overall average, which included .231 with a .302 on-base percentage as a pinch-hitter. He said those late, one-shot opportunities actually have helped him hone his hitting approach.

Mystery starter: Rockies manager Clint Hurdle did not reveal his starter for Sunday afternoon's finale of the four-game set against the Cubs, except to say the club is keeping its options open on bringing in a pitcher from outside the organization.

The Rockies signed righty Elmer Dessens to a Minor League contract and scheduled him to pitch for Triple-A Colorado Springs on Friday night. He can provide a veteran presence late in the season, but he won't be available on Sunday.

The Rockies are looking for a veteran starter with whom a non-contending team would want to part. A baseball source said Orioles right-hander Steve Trachsel, whose next turn to start is Sunday, is one of the names and is the type of pitcher the Rockies have discussed internally.

The opening occurred because right-hander Jason Hirsh was placed on the disabled list with a cracked fibula, which he suffered in his last start on Tuesday night.

Surgery for Lopez: The Rockies announced that right-hander Rodrigo Lopez will undergo surgery to repair his right elbow on Aug. 22 in Los Angeles at the Kerlan-Jobe Clinic. The operation will be formed by the Angels' medical director, Dr. Lewis Yokum.

Lopez has been diagnosed with a torn flexor tendon, and doctors will check for other damage.

Up next: Rockies right-hander Josh Fogg (6-7, 4.73 ERA) will start for the Rockies against Cubs left-hander Rich Hill (6-6, 3.54) in the third game of the four-game set on Saturday at 6:05 p.m. MT.