A federal judge in Philadelphia declined Tuesday to approve the proposed $765 million settlement between the NFL and players over football-related concussions, which have the potential to develop into debilitating brain injuries.

With an estimated 20,000 football players to be covered by the deal over the next 65 years, Judge Anita Brody was skeptical that enough money was being set aside.

"I am primarily concerned that not all retired NFL football players who ultimately receive a qualifying diagnosis or their related claimants will be paid," Judge Anita Brody wrote in her opinion.

Under the current proposal, the maximum award per player ranges from $1.5 million for Level 1.5 neurocognitive impairment to $5 million for amyotrophic lateral sclerosis or ALS.

Brody also objected to a clause preventing players from seeking damages from organizations not affiliated with the NFL such as the NCAA.

"A clause preventing going after damage recovery from an entirely outside party is surprising, and quite unusual to see in an agreement of this nature," said Marc Edelman, an associate law professor who specializes in sports law at the Zicklin School of Business, Baruch College/CUNY.

Edelman had initially supported the deal, believing it to be a manageable amount for the NFL and way for players to get immediate funds. But he said he hadn't realized the settlement included the clause Brody highlighted.

"I share Judge Brody's view that such an addition to the settlement was inappropriate, unnecessary, and does cause reason for concern," said Edelman.

As for the next step, Brody's opinion requested additional documents that would demonstrate that $765 million is enough. According to Edelman, both parties might appeal to have the ruling reversed, or begin negotiating a new settlement, among other options.