It really wouldn't be beneficial to replace poor performing starters with their reserves with the current engine. It works strictly off of ratings (which are multiplied by form and compared to opponent). In other words, the engine doesn't make players have "bad games." Momentum is not a part of the engine for individual players. If you feel your backups may actually be better or it is too close to tell, "let coach decide" will let the engine choose which player is better. However, it is very likely that the defender is simply better than your player and there isn't much you can do other than changing tactics or buying a better player.