PHILADELPHIA -- A federal judge denied preliminary approval of a $765 million settlement of NFL concussion claims, fearing it may not be enough to cover 20,000 retired players.

U.S. District Judge Anita B. Brody asked for more financial analysis from the parties, a week after players' lawyers filed a detailed payout plan.

"I am primarily concerned that not all retired NFL football players who ultimately receive a qualifying diagnosis or their [families] ... will be paid," Brody wrote in a 12-page opinion issued Tuesday.

The proposed settlement, negotiated over several months, is designed to last at least 65 years.

The awards would vary based on an ex-player's age and diagnosis. A younger retiree with Lou Gehrig's disease would get $5 million, those with serious dementia cases would get $3 million, and an 80-year-old with early dementia would get $25,000. Retirees without symptoms would get baseline screening and follow-up care if needed.

"We respect Judge Brody's request for additional information as a step towards preliminary approval," NFL spokesman Greg Aiello said in a statement. "We will work with the plaintiffs' attorneys to supply that information promptly to the court and special master. We are confident that the settlement is fair and adequate, and look forward to demonstrating that to the court."

Christopher Seeger, an attorney for the retired players, also said in a statement that he is confident the settlement ultimately will be approved.

"We are confident that the settlement will be approved after the court conducts its due diligence on the fairness and adequacy of the proposed agreement," Seeger said in the statement. "Analysis from economists, actuaries and medical experts will confirm that the programs established by the settlement will be sufficiently funded to meet their obligations for all eligible retired players. We look forward to working with the court and special master to address their concerns, as they rightfully ensure all class members are protected.

"We believe this is an extraordinary settlement for retired NFL players and their families, and have received overwhelming support as they have learned about its benefits. We look forward to finalizing this agreement so they can soon begin taking advantage of its benefits."

NFL commissioner Roger Goodell defended the league's $765 million concussion settlement in September, but U.S. District Judge Anita B. Brody denied preliminary approval of the plan Tuesday. AP Photo/Carolyn Kaster

Law professor Gabe Feldman, who directs the sports law program at the Tulane University Law School, called the ruling a setback but said "there's no reason to panic."

"The question remains whether this gives pause to some of the retired players and makes them question whether this is a settlement they want to be a part of," he said.

Some critics have argued that the NFL, with more than $9 billion in annual revenues, was getting away lightly. But the players' lawyers said they will face huge challenges just to get the case to trial. They would have to prove the injuries were linked to the players' NFL service and should not be handled through league arbitration.

Sol Weiss, a lead lawyer for the ex-players, also remained confident the class-action settlement will ultimately be approved.

"I am very confident that the [actuarial] people we used are right, and that there will be enough money to cover these claims for 65 years," Weiss said.

More than 4,500 former players or family members have filed suit, some accusing the league of fraud for its handling of concussions. They include former running back Tony Dorsett and Super Bowl-winning quarterback Jim McMahon, who suffers from dementia.

Layn R. Phillips, a former federal judge from California hired by Brody to lead settlement negotiations, had called the deal fair.

The settlement would include $675 million for compensatory claims for players with neurological symptoms; $75 million for baseline testing for asymptomatic men; and $10 million for medical research and education. The NFL also would pay an additional $112 million to the players' lawyers for their fees and expenses, for a total payout of nearly $900 million.

With the preliminary rejection in hand, the sides could try to assure the judge the money will last or raise the fund amount to ease her concerns. They also could try to start over.

Brody is expected to hold a hearing sometime this year at which critics could challenge the settlement terms. She would then decide whether or not to grant final approval. Retirees could opt out of the settlement and sue the league individually, but there is no guarantee they would ever get to trial or see any money.

The judge last month named Perry Golkin, a longtime executive at the investment firm Kohlberg, Kravis, Roberts & Co., as a special master to advise her on the case.

The players' lawyers filed a few hundred pages of documents and exhibits last week, but Brody asked for more analysis to bolster their claim that the money won't run out.

"Unfortunately, no such analyses were provided to me in support of the plaintiffs' motion. In the absence of additional supporting evidence, I have concerns about the fairness, reasonableness, and adequacy of the settlement," Brody wrote.

ESPN.com's Darren Rovell and The Associated Press contributed to this report.