When they are offically put on the DL. It sucks, you might not be able to do it till the beginning of the season.
All Public Leagues have one DL spot, while Custom League commissioners can choose to have up to five DL positions. Any player is eligible to be placed on the DL as long as he is on the real-life disabled list. Once on the DL, a player may stay there for the entire season, even if he returns to real-life action. However, once a player comes off the real-life DL, you will not be able to complete any transactions until you remove him from your team's DL. You will, however, be able to make intra-roster moves (switching players between bench and active) regardless of a player's DL status.