Odds are he plays for Miami next year, but this is in no way a done deal. Sophisticated parties represented by counsel (bad lawyers for Mensah) negotiated and executed an agreement. Mensah breached the agreement.
Normally, courts do not enforce these kinds of injunctions when monetary damages are easily calculable and appropriate. Probably that's the end result. But sometimes judges enforce the letters of agreements, especially when both sides are sophisticated and supposedly have lawyers representing them (as opposed to a consumer signing a form agreement with a big institution).
Duke could easily drag this out so it's not resolved for months.