In the old days, starters who were off form were shipped to the bullpen, to straighten out their act. Somebody out of the bullpen or from the minors would replace them in the rotation. Nowadays, the conventional wisdom is to "stretch out" guys in the minors to prepare them to start. And there is a sharp distinction between starting and relieving.
Baseball needs to take a good look at how pitching is handled these days--in my view. How it has evolved. A lot of things are done without much foundation.For example, you have the Sox starving for a reliever the last couple games, and one of their best ones, Workman, is sitting in Pawtucket being "stretched out" to start. Why a reliever shortage? Because relievers are often used in games to keep pitch counts down for starters--instead of using them when starters are actually in trouble, as they were used in the past. Overused. They can't do both.
With all the arm injuries occurring lately, the media should take a look at the new pitching orthodoxy, e.g., pitch counts, bullpen use,etc., and see what effect any of this has on durability and conditioning. There just didn't seem to be that many arm injuries in the old days. Raschi, Reynolds and Lopat pitched a lot of years in the Yankee starting rotation. So did Lemon, Garcia, Wynn and Feller for Cleveland.