http://iscoresports.com/

Is there a way to hide opponent players when I switch to a new season? Also, if I delete them, will it do anything to the games that are already completed and stored?

The recommended approach is to use Leagues. Put players you want to hide in a league like "Inactive Players" or something similar, and turn that league off, and the players will not be seen. Many players will return from season to season... different teams, but same players, so it is a good idea not to delete them so you can maintain a history of them.

Do not delete them! Instead hide them with leagues. (as ftm suggested)
You will be tempted to delete players to 'clean up' player manger. Don't. Instead move them to a special "hidden players" league..

I am not sure why, but sometimes after you delete a player in player manager they will no longer appear in the past games. If you save past games and periodically go look at them... Then don't delete players. The extra storage is negligible...

I learned this the hard way.

Thanks. I will give it a try.