A lot of people view Football as their refuge from the daily bull **** that surrounds us, which includes politics.
If I were a professional athlete I'd make sure to not get politics involved not just because I believe it doesn't mix well with sports but because if I took any sort of a stand, there is a good chance that I'd be alienating a decent segment of my fanbase.
If you want to use your high profile for a certain advocacy there are plenty of platforms that you could use to get your message across. Twitter, Facebook, interviews etc. Just keep it away from the sport itself, that would be my advice.