Contextual Filter / Relationship
a roster lists the players on 1 team, correct? If the view is at a url like roster/TEAMNAME you want a listing of players where the contextual filter on teamname provided by argument 1 in the URL, or else 404 or hide the view.

If the roster is not on a URL but say a block on 1 players page (showing the rest of the player roster for his team) then I would add a Views Field View to the player content type that uses the player's own team as the argument to the View I describe above.

Contextual filters are good when you provide the context for the view (like a URL, or another field). relationships just help you to get more information into your view (like additional fields).