I have noticed that if I try to use a pitcher as a reliever when they were a starter in real life, that they usually get rocked if sparky puts them in 2 days in a row. Is this a coincidence, or is part of the sim design to wncourage us to use players in the roles they actually filled in real life? I'm thinking it might be part of he "fatigued from recent activity" I've heard about. If so, is it more likely to happen to starters than relievers? And if so, why would Sparky put them in 2 days in a row?

That seems... unusual. It shouldn't make a difference that they are starters vs. relievers. Typically starters as relievers is a thing that works and is commonly used (see: Milaki/Chamberlain/Toney/Falkenberg/etc.), though ideally they'd play every second game.

This thread covers how the bullpen is used:
www.whatifsports.com/forums/Posts.aspx?TopicID=452075

Posted by dcmatcheck on 2/5/2013 7:25:00 PM (view original):
I have noticed that if I try to use a pitcher as a reliever when they were a starter in real life, that they usually get rocked if sparky puts them in 2 days in a row. Is this a coincidence, or is part of the sim design to wncourage us to use players in the roles they actually filled in real life? I'm thinking it might be part of he "fatigued from recent activity" I've heard about. If so, is it more likely to happen to starters than relievers? And if so, why would Sparky put them in 2 days in a row?

A player's or pitcher's fatigue number will tell you everything that you need to know.  If a player is at 100%, competition being equal, he is just as likely to perfrom one day as he is the next.

There is such a thing as appearance fatigue, but I have never seen this kick in unless a pitcher enters more than ~50% of his team's games.  The bottom line is that using a starter in a relief role doesn't seem to have any detectable drag on performance.