I can't figure out if the computer A.I. is substituting for my players in matches, or if there's even something I can do to force subs. I'm new ... had two matches so far ... in both, one of my starters got tired, and the commentary said this led to an opposition goal. I suppose I could just not start the guy until he's fit, but in the meantime, will the A.I. sub for him when he's knackered?
The only time there's a substitution in a Hattrick match is if there's an injury. There's no way to set one up for a certain time in the match. The way the game is set up, adding substitutions would require them to recode and redo a lot of things, primarily the way training and formation experience are handled. I'd love to see subs, though. Just don't think it will happen any time soon.
BTW, if your players are getting tired, it's because of low stamina. Excellent is really the highest stamina can go, and the only way to raise it is by training stamina. Players generally gain about 1 level of stamina per week of training, but not necessarily. Some players might not go up one week, but go up 2 the next. Stamina is most important for inner mids and wingers, but is somewhat important for all positions (I think).