Cabrera was off to a rough start, but is improving, with his last start his best yet. Unless they change their plans for him and make him a reliever, his starting berth is likely not in jeopardy. Remember that he is 22 years old with almost no Triple-A experience.
DellaValle has a 2.25 ERA, a .237 batting average against, a 28-to-4 K to BB ratio, and has averaged six innings per start, but has gotten almost no run support. If he had any, he would be 5-1, instead of 1-2. That is pretty darned good pitching, as far as I am concerned.