The 26-year-old Ely is no longer a prospect, but did have a monster season in Triple-A in 2012, winning the Pacific Coast League pitcher's Triple Crown.
As for Rasmussen, he continues to be desired by organizations because he's a lefty with a plus breaking ball. He's been expendable, however, because he's just 5'9" 165 lbs. and not everyone believes he can remain a starter.
I'm in that group, but not because of his diminutive size. He's actually been a relatively durable pitcher despite his non-traditional pitcher's frame. But his ceiling as a starter isn't terribly high because he doesn't miss a ton of bats (7.3 K/9 in the minors) and because he walks too many batters (3.8 career BB/9). In the bullpen, however, control can be more easily overcome and he could strike out more batters if his velocity goes up a tick as it does with some pitchers when they work in relief.
Most importantly, his ability to spin a quality breaking ball could make him more than just a middle reliever. It could give him a chance to be a lights-out relief option against left-handed hitters and if he does see his velocity increase by a MPH or two, he could even be a quality set-up man.
The Dodgers have not announced how they will use him, but just 23-years-old, it's not necessary that they give up on him as a starter just yet. Even if he ends up in the bullpen, the extra innings of experience will do Rasmussen good.