the GE has a rating system.. its rates every player based upon a large number of factors which include the players skills, how tired they are, foul situation and the depth chart. Players on the depth chart get increasing larger increases in their rating for being higher on the depth chart. Injury or fouling out obviously takes your rating very very low. The GE will look to the bench and put in the player with the highest rating... if someone is in the game obviously they can't be subbed in. So.... the GE will put in the best person available... usually that someone on the depth chart, since that modifier is so much larger than the rest... but if nobody on the depth chart is available, or if the person on the depth chart is really really bad for some reason (bad in the sense of combined skills, fouls and tiredness) the GE can put in someone else.
The problem with making sure the person on the depth chart is put in NO MATTER WHAT... is that you have to make allowances for the case when there is nobody on the depth chart available, and that gets complicated when you really consider all the cases you can have, especially when considering what to do with other position simultaneously and when players on one depth chart can be playing on another.