Instructions
Praise the player who is unsettled. You can do this by selecting "Private Chat" from the "Interaction" menu. Select any praise option, such as telling the player how essential he is to the team, to lower the chances that he leaves. Do this every chance you get.
Offer the player a new contract from the "Contract Offer" screen. Make sure the new contract offers the player a higher monetary amount than the old one to keep him happy and make him unwilling to change teams.
Win games. The more successful your team is, the less likely a player becomes unsettled or an unsettled player wants to leave. Train your players whenever possible; teach them a wide variety of advanced moves for use during games to help them win more often.