Sorry to ask what seems like a simple question, but is it possible to set a rule so that after a set number of minutes during a match, you take of the worst playing midfielder and replace him with the best available midfielder on the bench? It would seem a little strange it that wasn’t the case, hence why I’m asking :-).
Thanks.

No it isn’t possible. You need to select a position which you want to substitute, but you can substitute him by the best available player on the bench.
At this moment you could select a substitution based on the result at a specific time or you could select an event (injuries or red card).

Maybe you can repost it at http://www.rubysoccer.com/forums/3 as a suggestion Molinari?