Please check your in-game Inbox to see if you can find more details, as it will specify exactly what happened with that player.
There are several reasons why this could have happened:
1. You were in debt for more than 7 days, so the bankruptcy procedure was activated, meaning that the executive board started selling your best players until the debt was repaid 2. The player’s contract had expired and you did not renew it in the next 7 days, so he left the team 3. There were no free places for him when you bought the player as your Squad was full (28 players). After 7 days without signing him, he left. 4. You placed him on the transfer list or used the “Sell Now” feature.