Mögen doesn't only mean "to like." For example, it can have to do with the probability of something happening/being true.
Have a look at the following link, especially at what is listed under #4 and #5:http://de.pons.eu/dict/search/results/? ... &in=&lf=de
I think your sentence means something like:
"Maybe he was a tennis player."
"Perhaps he was a tennis player." ("Perhaps he did play tennis.")
"It might be true that he was a tennis player."
Serve and volley, serve and volley,