There's actually a thread on fatigue on the front page but the long and the short of it is you the sim uses PA for fatigue, not games. You'll be able to get 152 PA out of Tenace (110%) for the season without facing a fatigue penalty.
And it wasn't that Sisk balked in a run, it was EVERYONE else in the bullpen was pretty good, he just wasn't. I remember being a little kid and just begging Davey to go with McDowell, Orosco, Aguilera, Leach, ANYONE other than Sisk.