The National Football League's $764 million concussion settlement with retired players was dealt a setback Tuesday when a judge declined to endorse the deal, citing concerns that the amount might not be enough to cover all potential recipients.

The ruling puts on hold—but doesn't derail—the deal that the NFL reached last summer with thousands of former players who sued over concussion-related issues. The plaintiffs' complaint alleged that the league concealed risks of head injuries and ignored growing evidence of the long-term...