Starters are expensive though. There are scenarios, yeah, but it feels like gilding the lily. They already have enough arms to go with a six-man rotation, most of whom are still better than most teams' third starter (not Walker, but Sanchez, and Turnbull if he comes back healthy and Phillips for as long as he can get away with it).
Now, we'd really be talking if they did put Turnbull into another trade, or figured out a way to offload Walker's contract without having to include an extra prospect (just money).