Legally you could if you a) prove willful intent of lying, and prove financial losses as a result and/or C) suffer mental anguish, embarrasement or humiliation. As an athlete, the only real financial losses could be loss of endorsement or contract dollars if either was terminated as a result of the accusations, or that he couldnt find a new contract because of it after his current one expires. But without provable financial losses, then a civil suit is pretty much useless. The mental anguish, embarrasement or humiliation is also hard to win on since most states have defamation laws that say being accused of a major crime will come with damage to your reputation regardless of whether they are true or not.... so harm to your reputation is presumed.
Since she published the accusations on instagram though, she has entered libel territory rather than just slander. States do handle those differently from one another depending in the situation