The Rockies' bullpen was used heavily during their recently completed 10-game road trip. De Los Santos (0-1) was unable to get past the fourth inning in either of his starts, and Jorge De La Rosa (5-6) was lifted after giving up seven runs in 1 1/3 innings on Friday against Florida.
"We needed another arm in the bullpen," manager Clint Hurdle said. "We've pitched a lot of innings out of the bullpen the last seven days."
Register, 25, went 5-2 with 16 saves and a 3.28 ERA in 48 games with the Sky Sox. Register was originally selected by Colorado in the third round of the 2004 First-Year Player Draft, but was taken by the Mets in the Rule 5 Draft on Dec. 6, 2007. He spent Spring Training with the Mets, but was returned to Colorado on March 27.
Register's stay with the Rockies will likely be a short one -- as in 48 hours. Left-hander Jeff Francis, who has been on the disabled list since late June with inflammation in his pitching shoulder, is expected to be reactivated on Wednesday.
Register said he's looking at this as an opportunity to warrant a more permanent stay in the future.
"I just want to come in, and if I get a chance to get in there [Monday] or whenever, just throw like I know I can, and hopefully everything will work out," he said.
Register did get a chance on Monday, hurling one-third of an inning and striking out one while not allowing any runs.
Jeff Birnbaum is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.