Brothers, who spent a year as the Rockies closer in 2013 (posting 19 saves), spent a fair amount of 2015 in the minor leagues. In the major leagues with Colorado last season, he made 17 relief appearances covering 10⅔ innings with a 1.74 ERA, but eight walks and only five strikeouts. He's always had some control issues, with a lifetime BB/9 ratio of 5.2, though he's struck out 10.3 per nine innings over his career.
Heading to Colorado is 18-year-old lefthanded pitcher Wander Cabrera, who has yet to pitch in the US minor leagues. He spent the summer of 2015 in the Dominican Summer League, so he's a project for the Rockies.
Colorado appears to have jettisoned Brothers -- he was designated for assignment a few days ago -- in a money-saving move. Brothers is in his first arb-eligible year in 2016 and would likely be in line to make somewhere around $4 million or $5 million, probably too much for the Rox to handle. The Cubs can certainly pay a pitcher like Brothers that much. He could compete for the 25-man roster spot that was held at times in 2015 by Zac Rosscup.
This move leaves the Cubs' 40-man roster at 38. Looks like he'll have to change that number, though, if he makes the Opening Day 2016 roster.