Substitutions

I watched Valencia-Real Madrid tonight and noticed how the introduction of Sergio Ramos and Robben changed the game. Robben was twice as quick as anyone else on the pitch and eventually set up Real Madrid's second goal with a beautiful play down the flank.

I was thinking about what an impact substitutes can make in FM.

When they join the game, substitutes usually have a condition of about 20% better then starting players. I don't really care about player conditions, I only subsitute my players when they are about 65-70%, or if the next game is coming only 3 days later, 70-75%. Does this +20% mean an advantage for them or doesn't it really matter? Is there a script in a game that makes substitutes play better, just like players encouraged by team talks?

So if a key player is not doing particulary well, should I give the chance to his backup, who has worse skills but is less tired?