https://sports.yahoo.com/trump-attacks-nfls-massive-tax-breaks-twitter-130318649.html
The quirk for the NFL is that tv revenue goes to the league (association), and then is meted out to the times (separate legal entities) while expenses are that the team level. Ultimately the team owners pay tax on their income (or utilize losses.)
Potentially trump could try to tax the league (association), which wouldn't have the player salary deduction to offset income - that's not really a "loophole", since the owners/league incur those salaries to generate the revenue (matching of income/expenses.) If trump were to get legislation enacted in that regards, the ownership group would restructure themselves to avoid the double-taxation.
There is no loophole (other than potentially writing off asset purchases - which trump wants to expand.)