You can never lose more than your stake. Traders try to buy when Footballers are undervalued in the market with the aim of selling them at a profit. Buy low and sell high is the trading mantra. 

If your Footballer looks in danger of being relegated from the 1st Team (two footballers drop out of the tradable market each week) your shares will certainly be worth less, if not valueless, and you will lose part if not all of their value.