Rusin, 29, is seeking a spot in the starting rotation after going 6-10 with a 5.33 ERA in 24 games, all but two of them starts, last season. It was by far the most action in a season for Rusin, who went 4-9 with a 4.97 ERA for the Cubs in 24 games (20 starts) from 2012-14.
Carle went a combined 14-8 with a 3.68 ERA in 27 starts with Double-A New Britain and Triple-A Albuquerque last season. The Pirates took Carle in the 10th round out of Long Beach State in 2013.
Musgrave, who amassed 136 combined strikeouts in 147 1/3 innings last season at Class A Modesto and New Britain, was an eighth-round choice out of West Virginia in 2014.
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The situation becomes intriguing because Rusin is out of Minor League options. Most forecasts have the rotation consisting of lefty Jorge De La Rosa and righties Chad Bettis, Tyler Chatwood, Jordan Lyles and Jon Gray. However, Rusin threw two complete games last season and had some of the Rockies' best starts.
The Rockies are preparing Rusin as a starter, but they often carry eight relievers -- one more than most teams. Rusin could be used as a lefty complement to righty long reliever Christian Bergman.
Also, righty David Hale, who also hopes to have a say in the starting-rotation race but can pitch out of the bullpen, threw 10 pitches off a mound to a catcher -- who was standing rather than squatting -- on the sixth day since suffering a right hamstring injury in a running drill. Hale has stayed on his throwing program, and should be able to slide into game work once cleared from rehab.
Thomas Harding is a reporter for MLB.com. Follow him on Twitter @harding_at_mlb, listen to Podcasts and like his Facebook page. This story was not subject to the approval of Major League Baseball or its clubs.