One part of the answer to your question is based on their IP/G.
DoctorKz said: "My opinion is relievers with 1 inning or less stamina are overvalued. They fatigue quickly, take longer to return to 100%, and are not great at pitching back to back games. There are exceptions."
This is such an important point to understand about this game. Modern relievers with low IP/G are very very hard to use.
Here's an interesting example. Compare 1992 Dennis Rasmussen (the Cubs version with 43 innings) with "Moneyball Made Me Famous" 2002 Ricardo Rincon (the Indians version with 56 innings).
Both are lefties. Similar stats: WHIP+ Rasmussen 137, Rincon 134. WHIP# Rasmussen 0.97, Rincon 1.03. OAV+ Rasmussen 116, Rincon 115. OAV# Rasmussen .223, Rincon .229. HR/9+ Rasmussen 154, Rincon 171. HR/9# Rasmussen 0.43, Rincon 0.47. Pretty comparable. Maybe Rasmussen is a touch better.
Now look at their performance histories: Rincon has a .304 OAV, 1.64 WHIP, 5.98 ERA. Rasmussen has a .271 OAV, 1.36 WHIP, 3.98 ERA. Rasmussen has also been used almost 1200 times, which tells you something about how much more highly he is valued.
Why? Look at their IP/G. Rincon's is 0.79. He's one of those modern-era lefty relievers that in real life was typically brought in to face 1 or 2 batters with the platoon advantage. Rasmussen's is 5.3. He was basically a starter who was used occasionally in relief. In SLB, if you draft Rincon you pretty much have to keep his pitch count under 15. Personally I would set him to 5/5 or 5/10. He's good for 1 batter, tops. Then he will get tired rapidly. At 20 pitches, SLB is going to treat him like a pitcher who has been worked twice as hard as he can handle; his in-game fatigue will decline rapidly and his performance will plummet. I guarantee you many owners have used Rincon with a pitch count of 15/15 or 15/20. Bad idea.
Rasmussen can be set to a much higher pitch count (probably as high as 70-75). I wouldn't actually set him that high, because if he pitches that much he will be tired and unusable for several subsequent games, but during the game itself he will not fatigue anything like Rincon. I am convinced that one of the biggest mistakes newbies make, and one of the major reasons why modern day closers often severely underperform, is that their pitch counts are set way too high.