What you should say is yes, not taking proper care of your body while being a paid professional for a football club is wrong. You are paid to play for the team, and whether it be fasting or getting drunk before matches, it's simply not professional. One can concoct any stupid reason they want, but if it's a stupid religious reason for fasting, not showing up for matches, or other absurdities, it just has to be respected because it's "religion."
If people want to practice their religion, do it on their own time, not the my time or your time or the team's time. What Sissoko is doing by fasting is basically forcing his religion on the team, and that is unacceptable.
I love Sissoko and all, but he should refuse to play and take a pay cut if he wants to practice his religion on the team's time.