The deal, first reached in August, set aside up to $5 million (£3.05 million) for each former player diagnosed with a certain brain condition as a result of their years on the playing field.
Given the large number of people who could benefit from the suit - more than 4,500 former players signed on - US District Judge Anita Brody warned that the proposed deal may not include enough funds to meet the claims.
"I am primarily concerned that not all retired NFL football Players who ultimately receive a qualifying diagnosis, or their related claimants, will be paid," Brody wrote in papers filed in federal court in Philadelphia.
"Even if only 10 percent of retired NFL football players eventually receive a qualifying diagnosis, it is difficult to see how the monetary award fund would have the funds available over its lifespan to pay all claimants at these significant award levels," the decision said.
Up to 20,000 former players could ultimately be eligible for payment, Brody said.
The lawsuit, filed in 2012, contended that the league hid the dangers of brain injury players faced while profiting from the sport's sometimes violent physical contact. When the settlement was first disclosed in August, sports business experts described it as a modest amount of money for the NFL, which is believed to generate total annual revenue of $9 billion (£5.48 billion) or $10 billion (£6.09 billion).
In recent years, there have been suicides among current and former NFL players, including Jovan Belcher, Junior Seau, Ray Easterling and Dave Duerson. While none of those deaths could be directly connected to the sport, violent or erratic behaviour is consistent with symptoms of a condition tied to repeated hits to the head many players endure in games and practices.
A growing body of academic research shows those hits can lead to a condition known as chronic traumatic encephalopathy, which can lead to aggression and dementia.
The research has already prompted the NFL to make changes on the field, including banning the most dangerous helmet-to-helmet hits and requiring teams to keep players who have taken hits to the head off the field if they show symptoms including dizziness or memory gaps.
- American Football
- Sports & Recreation