if it's not a の-adjective, what do i have to consider it in the sentence? Do I read '二人の選手' as 'athlete/player of two persons'? If it's a の-adjective i think i can read it as 'two players' or 'pair of players' right?
Japanese works differently than English. の is used to join two nouns, with the first modifying the second.
book の reader → the book's reader, the reader of the book
dog の keeper → the dog's keeper, the keeper of the dog
Sometimes the relationship works out a little differently than the above, in which case you must rely on context and the meaning of the individual words.
For your sample phrase, we have the noun 二人【ふたり】 and the noun 選手【せんしゅ】. The の in between could be possessive, wherein the 二人【ふたり】 and the 選手【せんしゅ】 are distinct and separate entities:
two people の player(s) → the two people's player(s), the player(s) of the two people
Alternatively, this could mean that "the two people's player(s)" means the two people are the players. This is most likely the correct interpretation in your sample.
A similar phrase might be:
flower(s) の bunch(es) → a bunch of flowers, where the flowers are the bunch
While の is often used for the possessive, remember that ultimately it's used to join two nouns, where the first is modifying the second.