In a major blow to the N.F.L., Colin Kaepernick achieved a preliminary but important win in his case accusing the league of colluding to keep him off the field because of the player protests during the national anthem that he instigated.

The ruling, essentially granting a full hearing on the dispute, keeps alive a case that the N.F.L. desperately wanted to go away. The league is preparing for a new season beginning next week and is still grappling with how to defuse the smoldering debate over players who demonstrate during the national anthem to protest racism, police brutality and social injustice.

Although the number of players who kneel has varied — and dwindled over the course of last season — since Kaepernick first did so in 2016, during a wave of police shootings of African-American men, the issue continues to divide fans and vex owners. It has also inspired persistent tweets from President Trump, whose calls for players who kneel to be fired has put pressure on owners, many of whom support him.

Kaepernick, once one of the league’s best quarterbacks, has been out of work since March 2017, when he became a free agent before the San Francisco 49ers could release him. As a parade of lesser quarterbacks, at least statistically, found work, he filed a grievance asserting that the league’s owners had conspired to keep him out because of his protests.