I think difficulty levels should be in.
However, I do not think they should have any effect on the match engine at all - it would be hard to code, lead to bugs and plain wouldn't work. However, they could be in without changing any in game calculation - just get the game to hold users hands a bit more.
For example: You are in match and the assistent comes up with advice relating to us losing the ball too often due to poor short passing, and the user should play more direct.
Very easy - the message comes up as (say we are man utd vs chelsea): 'Our passsing has been poor today. In particular, we have misplaced far too many short passes, especially from our centre-backs. To solve this you should play a longer, more direct passing game, making sure our centre-backs are told to quickly clear the ball upfield.'
Easy: 'Our passsing has been poor today. In particular, we have misplaced far too many short passes. To solve this you should play a longer, more direct passing game'.
Normal: 'Our passsing has been poor today. In particular, we have misplaced far too many short passes.'
Hard 'Our passsing has been poor today.'
Very Hard: Assman advice disabled.
This could be implemented in many areas of the game, from choosing your club, to making your tactics, signing staff, picking your team, transfer negotiations, and many others. On very easy, you will be practiclly told what to do, good for those who know little about football and need lots of help at the start. On very hard, you will get no advice at all - it is all down to your judgement. One other key point - on the easier difficulties, the game should help you get a very good assman (maybe even a perfect one on v.easy) so that he won't give you nonsence advice, even if you are in the BSS/N.