Rockies shortstop Troy Tulowitzki has been placed on the 15-day disabled list because of a strained left groin, the team announced before tonight's game against the Astros. Infielder Chris Nelson has been activated to take Tulo's roster spot. Tulowitzki underwent an MRI today, and the Rockies expect to know the results some time tonight. While it's in the same area, this is a different injury than the one suffered in the second game of the season. "In the best-case scenario we were trying to milk it for five to six days, and I just don't think that is smart right now," manager Jim Tracy said. "I don't want him to have to play careful. If he tears a groin, you're looking at being out for two and a half months."