[Networking] Player prefab in a Hearthstone-like card game

Hi guys! I'm kinda new to the networking.
I'm working on a card game like 'Hearthstone' or 'Yu-Gi-Oh'. I would be a LAN multiplayer game. I already done with the game logics, I can summon cards and fight them, so I decided to create it's multiplayer version.
When I create a 'NetworkManager' GameObject, and assign the NetworkManager script to it, it asks for a player prefab. So anyone knows what is it in my case?
The game is controlled by the mouse, and there is not really 'player'. There are only cards.

Thank you in advance.