PHILADELPHIA — The NFL and former players must try to negotiate a dispute over whether complaints about concussion-related injuries belong in court or in arbitration, a federal judge said Monday.U.S. District Judge Anita B. Brody of Philadelphia had planned to rule July 22 in a legal fight that involves about 4,200 former players and could be worth billions of dollars.But instead she ordered the two sides to begin mediation with retired federal Judge Layn Phillips. The retirees want the right to sue the league, while the NFL insists the claims must be arbitrated under terms of the collective bargaining agreement.Brody asked for a progress report by Sept. 3, while placing a gag order on the lawyers. Both sides agreed to comply.Many former players say they suffer from dementia, Alzheimer’s disease and other neurological conditions, which they believe stem from on-field concussions.