Don't buy a new player, as it seems they never seem to perform right away. As for your opponent he doesn't watch the matches, which is why changes are not made. But be careful as it is the final and he might decide to watch. I would say post a completely different formation then what you would usually play with to try to get him to change his formation early. Then switch back right before. Otherwise you never know what will happen in a final. 2 seasons ago one of my players got a red card with the game tied, and we scored with a minute left, so you just never know! Good luck though!