Pause the game, then go down to Strategy (i think its strategy) and go to depth chart. Have you never played any of the Madden games?
I must be stupid or somthing but I can not figure out how to sub players in and out in Madden 08. please help
Stradegy--> Depth Chart--> Locate your new player's position and make sure he's at the top of his position. Are you seriously new and unexperienced with Madden?