Hello, Knight165. I have admired your work with this game the short time I have been an MLB The Show player. Just tell me how and what needs to be submitted and I will do my best to comply. My PS3 is not usually online, but can tether it if that is needed. I would love to help out.
Answering your question: When the LRP is left empty, the AI will use your MRP when your starter is knocked out early or has to leave a start early because of an injury. If you this spot is occupied by a pitcher who is "stretched out" with the same stamina as your 4th and 5th starters (in my scale, stamina ratings of at least 60-69), he will have the best chance of being effective, as the AI will treat him as he would treat the departed starter. This includes letting him bat for himself in non-DH games. If he is effective, the cpu (AI) will let him pitch into the 6th inning in DH games if he came in with 100% rest. In non-DH, games the cpu will let him bat in bunting situations and low leverage at-bats (no one on, 2 out, for example). If he wasn't in the LRP slot, the cpu would replace the early exiting starter with a MRP and he will pinch-hit everytime the pitcher's spot in the order comes up or attempt to double-switch. If this goes on from, say the third inning on, you will go thru your bullpen, with some asked to pitch multiple innings. This is with starter stamina sliders=4, reliever stamina=0 and manager hook=5. Putting together a few of these occurrences will fatten up the IP of your MRP. In my test using the common practice of a LH and a RH in the setup slots and LRP left empty, some MRP would compile 100+ innings in season sims, which is very 1970s-80s. Everybody would have more innings than appearances with the same slider settings explained above.
Using this method for setting up the bullpen, my contending teams very much look forward to September when the rosters expand. Then, you can max out all 3 LRP slots, plus utilize both SU slots. This cpu will use only the most rested pitchers first in the long relief slots when needed for early KOs and those long extra-inning affairs. But the entire pen gets a boost as you proceed on your stretch drive and prepare for the postseason. Now, with this bullpen setup, combined with the slider setting, you will see MRP pitch multiple innings in some situations. But you will also see the cpu parade reliever after reliever during close game situations in the 7th, 8th and 9th innings. So your MRP will have appearances where the might pitch 1.1, 1.2 or 2.0 innings. But the will also have appearances where they will be yanked without retiring a batter or with 0.1 or 0,2 innings pitched. During the course of the season, it balances out to pretty much to realistic innings pitched numbers of today's baseball. I have also tried slicing the reliever stamina rating on certain LH pitchers to try an replicate the popular LOOGY reliever. My usual minimum for a MRP/SU/CL is 28, max 35. I slashed some to 15 to get the cpu to allow them to face one batter. But that experiment didn't work because the cpu would not bring the LOOGY into the right matchup, LHP v LHB, all the time. But he would only face the one batter. Also, with the stamina rating so low, the cpu would not pitch him on consecutive days.
I hope this helps and gets into the proper hands to make a difference.