Pitcher's Fatigue
Pitcher fatigue/rested-ness ratings slide between -3 and 13. You'll never see a -3 on your reports because pitchers recover somewhere between 3 - 5 points per day, which means that by the time you receive your reports, any pitcher at -3 has already recovered at least to 0. Recovery rates of pitchers depend on several factors, the most important of which are the average number of real life batters faced per outing, and the hook rating you apply (a longer hook encourages a pitcher to recover at a slightly faster rate), and, for closers, whether you have designated a "preferred closer" (preferred closers recover at a faster rate because they are usually asked to pitch only 1 inning or so per outing).
Once a pitcher recovers to a 10 his rate of increase slows dramatically. With an extended layoff he may climb all the way to 13, but once he tops 13 he reverts to a 10, the idea being that a pitcher who goes too long without pitching becomes less effective rather than more.
A pitcher whose rating exceeds 10 upon entry into the game naturally has better probabilities than a pitcher who enters at 6. Keep in mind, however, that some pitchers at 6 are better than others at 13). Here is the formula used to adjust a pitcher's opponent batting average for fatigue:
( ( 85 / ( F + 75 ) ) * OAV )
F is the fatigue rating and OAV is the pitcher's opponent batting average. A completely fatigued pitcher (at rating 0) with a base OAV of .250 would worsen to .283 while an over-rested pitcher (at rating 13) would improve to .241. Obviously it pays dividends to have rested pitchers, but the impact is somewhat mitigated from what you might suppose.
Bear in mind also that there are many other factors that impact a pitcher's OAV. For example, a fully rested pitcher (at rating 10) entering with a .250 OAV really enters lower than that, because every outing is plotted along a "work curve" along which the pitcher enters "stronger" than his mean, gradually regresses to his mean and then slides below it. This curve is skewed slightly in the pitcher's favor to compensate for the fact that most pitchers will occasionally be required to pitch before they are full strength. Obviously it matters, too, whether batters are right-handed or left-handed, whether the pitcher is at home or on the road, whether the infield is drawn in, whether the pitcher has exceeded his RPL etc. The overall provability structure is quite complex; the intent here is only to isolate the effect of fatigue/rest on pitcher performance.
In general, it is wise to make sure that you have sufficient depth in your pitching staff. There is no magic number of pitchers needed. It depends on what kind of pitchers you have, whether they are capable of longer or shorter outings, the number of men in the rotation and so on. If your pitchers must continually enter at less than full effectiveness it will certainly have an erosive effect on your pitching staff. On the Team Diagnostics report you can see the average strength of your starters and relievers upon entry into the game. When this number falls below 10 it may indicate the need for more depth. Note also that you can control the minimum fatigue level from the Pitching Roster screen.