De Los Santos (0-0, 4.50 ERA), who hadn't pitched in the Majors since 2005 and hadn't started a game since 2000, walked six in four innings on Monday in an 8-4 loss to the Pirates. He was called up from Triple-A Colorado Springs for the start.
Manager Clint Hurdle held off on announcing De Los Santos as Saturday's starter until after Thursday afternoon's non-waiver Trade Deadline, just in case the Rockies made an acquisition.
If De Los Santos' problems were because of adrenaline, he has a chance to show what he can do under less emotional circumstances.
"I heard him say he was going to be a much better player, and what player wouldn't be much better after a six-walk performance," Hurdle said, smiling. "I do feel, though, he will be better.
"I don't think that's an easy transition, being where he's been, what he's gone through [including time in the Mexican League], to get into that spot and have fun, not make too much of it. But the human side is going to get involved."
Unless the Rockies find a pitcher through the waiver process -- players can be traded after they clear waivers but must be on a team's roster by Aug. 31 to be eligible for the postseason -- in-house options must perform well. That's what happened last year, when veteran Minor League pickup Mark Redman and callups Ubaldo Jimenez and Franklin Morales all helped the club get to the playoffs and the World Series.
O'Dowd noted that left-hander Jeff Francis should return as early as Tuesday from left shoulder inflammation. He made his third and, the Rockies hope, final rehab start at Double-A Tulsa on Thursday night.
O'Dowd said Morales, sent down to Colorado Springs in April because of control issues, is making "small progress" in his recent starts and could be a factor before the season ends.
Right-hander Jason Hirsh, a rookie last year, hasn't pitched in the Majors this season because of a rotator cuff strain. Hirsh is having a hard time with consistency at Colorado Springs. O'Dowd said the Rockies "still like him," but it might be tough for him to contribute this year.
Thomas Harding is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.