Right-click on the player, in the drop-down menu select "move to". There you should see options "Move to" > "Reserves", "Under 19", along with "Make Available To Reserves".
This will make the player available for selection in the reserves for the next 5 days without having to "drop" him down to the reserves.
It's a nice feature, but a little clunky, as short of physically dropping the player to the reserves and risking him getting unhappy, you have to do this every five days. A check option would perhaps have been easier
IMO :thup: