While not expressly linked, the move may very well be in reaction the announcement yesterday that reliever Manuel Corpas, will be undergoing arthroscopic elbow surgery to remove bone chips from his elbow.
Chacin, who was posting a 3.14 ERA in 18 starts, is expected to be a future starting pitcher for Colorado, and has developed major-league command of a sinker and changeup to the point where the front office deemed him ready to hit the Majors. His solid work in AA combined with his strong showing in Spring Training made him a prime candidate for making the jump to Coors.
In 106 2/3 innings pitching this season, Chacin is sporting a 54% GB% as well as a 2.21 K/BB ratio.
SBNation blog Minor League Ball covered Chacin as a transition candidate here.
While this may be the most exciting move recently, the Rockies may yet make other, and are still linked to the Orioles' Danys Baez and the Indians' Rafael Betancourt. Randy Flores, Joel Peralta, and recently signed Matt Herges are also available for callup.