He signed a temporary contract, willingly, for which he is highly paid and comfortable.No because slavery doesn't entail earning double a nurses annual salary on a weekly basis.
Anyone who has any 'sympathy' for a footballer being forced to adhere to their long term contract that they willingly signed is a complete idiot. Football isn't a normal job. Footballers are remunerated quite substantially in order to be controlled by their employers.
This is not slavery.