We know players will get a rating while match is playing.

But since now, we cant let the player who is getting a low rating out of the field, So I suggest the system could add more conditions about rating. Such as "player out rating is lower than 4" etc..

Is it possible?

very good idea! this make the match more nice!

But then it ruins the unpredictability of youth, there is supposed to be that element of suprise when picking youths you know you're taking a risk in that they could play like a 3.5 star player or a 5.5 star player if they had that youth bonus, so I disagree.

The unpredictability of youth is why this is a good idea. It means you can take a risk on a player, but if he doesn't perform well you can sub him out.

More conditions for substitutions would definitely be a good idea, and also the possibility to make a substitution to change your tactical setup. I think it would be a very useful addition to be able to switch to a more attacking formation if your defensive approach doesn't pay off....or switch to a more defensive one if you take the lead against very strong opposition. It could add a tactical aspect to the game.