Both sides in the stalemated National Football League labor negotiations yesterday were bracing for the second players' strike in six seasons, at the end of tonight's game between the New England Patriots and the New York Jets.

There was no progress toward a settlement. The NFL Players Association and the Management Council, the owners' negotiating arm, have not met formally since Tuesday, and no further talks are scheduled before the strike deadline. The top negotiators did not communicate with each other yesterday and have no plans to do so today, they said.

Jack Donlan, executive director of the Management Council, said he saw no way to prevent a strike unless someone "could pull a rabbit out of a hat." He also said the Management's Council six-member executive committee, headed by Tampa Bay Buccaneers owner Hugh Culverhouse, will meet today in New York.

"Today is a day when everyone is sort of getting prepared for the ultimate deal, and that's a strike," said NFLPA Executive Director Gene Upshaw from union headquarters here as the union put the finishing touches on signs for picket lines and began instructing player representatives on picketing procedures.

During the 57-day strike in 1982, the owners locked out the players. This year, management intends to continue fielding teams with players cut in training camp, other free agents and any current players who decide to cross picket lines. The league has told the television networks that, in the event of a strike, this week's games will be postponed until the end of the season and play will be resumed Oct. 4.

As of late last week, teams had commitments from between 10 and 40 replacement players to report in case of a strike, according to a team executive.

Upshaw said players who report to the practice facilities run the risk of not getting paid if no games are played, and he again asserted that not all clubs will be able to field teams. Management Council spokesman John Jones said his side will not comment on contingency plans unless there is a strike.

Meanwhile, the Management Council has written each of the union's 1,600 members, offering improvements to their Sept. 7 contract proposal and asking them to ask their team's player representative to reconsider Tuesday's strike deadline, according to Upshaw.

The letter, under the heading "NFL Update '87." concludes: "The leadership of the players' union has the authority to stop the clock on the strike deadline and continue negotiations. Urge your union leadership to give the negotiation process more time."

Upshaw dismissed the letter as "not being worth the paper it's written on."

Less than 24 hours earlier, Upshaw rejected Donlan's request to extend the strike deadline 30 days, a move Upshaw claims would enable the owners to receive another $98 million payment from the television networks.

A strike would cost both sides. Although their payrolls would be reduced significantly, the owners would face ticket refunds and television rights fee rebates. The 1982 strike cost the players approximately $5,000 per man a week; a strike this year would cost them $15,000 per man per week, according to the NFLPA.

Despite these prospects, there has been little movement in the negotiations since opening offers were made April 20. Each side claims the other doesn't want to negotiate, and the issue of free agency is the major roadblock. The owners say it is the only issue preventing settlement; the union says it is one of seven "horizontal" issues.

The owners have offered to liberalize the free agency system, but want to keep its two main elements: compensation and right of first refusal. The players originally asked unrestricted free agency for all players, but, in their latest proposal, offered to allow right of first refusal to a team to keep players lacking four seasons in the league.

Some in management can't understand why the players would strike over unrestricted free agency, an issue they say would affect only two or three players per team.

"I don't even think they {the players} know what they're striking for," Washington Redskins Coach Joe Gibbs said in a postgame interview on WMAL Radio. "I don't really think they want to strike."

Pittsburgh Steelers owner Dan Rooney, a member of the Management Council's executive committee, said in an interview on CBS television yesterday that one reason the owners are dead set about changing the current free agency system is to protect themselves from one another.

In baseball, some free-spending owners got into the free agent market a decade ago, and salaries shot up. The signing of free agents in baseball has decreased dramatically the last two years, and the Major League Baseball Players Association has filed two grievances against the owners, charging collusion. An arbitrator is supposed to announce his decision on the first of those grievances today.

One high-ranking executive said the owners have concluded among themselves that, if they modified free agency, all owners except the Los Angeles Raiders' Al Davis would exhibit the fiscal restraint shown by baseball owners the last two years.

The union has been predicting a strike since last winter. In Game Plan '87, the NFLPA outlined what the owners' strategy would be: to stall negotiations and try to divide the union. According to Upshaw, that has happened in the form of no negotiations, the threat of free-agent games and the owners' threat of no lump-sum bonuses that were part of the 1982 settlement.