You can see my other thread for examples, but as for my take on fatigue, defense will degrade fastest (but not truly noticeable until under 93%), hitting degrades slowly and not truly noticeable until in the 80s (maybe even the low 80s) and pitching is similar with the true studs (sub 1.00 WHIP and great normalized stats) not showing degradation until the mid-80s and lesser pitchers below 93%, with BBs, HRs and a loss of Ks being the primary signs.
That said, I try to keep my guys at 100% at all times still unless I need to push for the playoffs. Then I feel comfortable going with fatigued guys as typically my bench players/Long B bullpen types are not nearly as strong as the starters.