It’s obviously inferred and implied. It’s an unenumerated imperative similar in concept to unenumerated rights via the 9th Amendment to the Constitution. You know, things that aren’t explicitly spelled out rights but are Constitutional rights nonetheless, like the right to travel, right to vote, right to marry, right to privacy etc etc etc. Not all moral imperatives are spelled out in a contract and they don’t need to be. There are inferred responsibilities that come with job requirements and this is overtly obvious and wanting to win at the highest level is part of the job requirement for a professional athlete. If, as a professional athlete, you don’t care about winning, then get off my team.