The National Football League yesterday suffered the biggest blow to its prestige since the bidding war in the 1960s when All-America linebacker Tom Cousineau signed with Montreal of the Canadian Football League.

The Buffalo Bills had selected Cousineau, of Ohio State, No. 1 in the NFL draft in May. A source in the Bills' organization said, "It is our understanding that the deal involves ownership in the Canadian team for both Cousineau and his attorney, Jimmy Walsh," who negotiated Joe Namath's onetime $450,000 salary with the New York Jets.

But later, at a news conference in New York City annoubcing the signing with Montreal, Cousineau said, "That's not the situation for me." However, Walsh admitted he and Namath are negotiating to buy into the Alouettes.

A spokesman for the Buffalo club said, "The Bills made Cousineau a contract offer in excess of any ever signed by a No. 1 draft choice in the NFL, including Earl Campbell, Tony Dorsett and Ricky Bell."

Those running backs reportedly earn about $250,000 a year. Montreal's clincher was described as a "multiyear" contract. The Bills' offer was believed to cover five years.

All-purpose running back Terry Metcalf, who played out his option and defected from the NFL St. Louis Cardinals to CFL Toronto for the 1978 season, received a seven-year, $2.1 million contract.

A few weeks ago, only about half of the NFL's first-round draft choices had been signed, but yesterday there was evidence that negotiations had been stepped up with the opening of training camps. Only offensive tackle Jon Giesler of Michigan, Miami's top pick, is reported to be still unsigned.

Cousineau said, "It was the best move for me. I'm of French descent. Montreal is the best place for Tom Cousineau to go. I've played with great players and held my own. I can play in the NFL."

Tex Schramm, general manager of the Dallas Cowboys, said of Cousineau's attorney buying Alouette stock: "There would be a conflict of interest if the Alouettes wanted to trade Cousineau. I don't think the NFL would approve a deal like that."

He likened Montreal to George Allen's big spending as a coach at Los Angeles and at Washington in the NFL.

"He (Allen) gave away everything but the club," Schramm said. "Now they're (in Montreal) giving away the club. I don't know anything about the franchise; maybe he (Walsh) is getting a piece of nothing.

"Nobody (American players) stays very long in the Canadian league.

"Johnny Rodgers and Anthony Davis didn't. Maybe Cousineau will enjoy playing there, but he's not accepting the challenge of the major league (the NFL)."

Jim Murray, general manager of the Philadelphia Eagles, said of the deals by Cousineau and attorney Walsh, "It's startling...amazing, amazing.

"McDonald's (the fast-food chain) turned up the price of Canadian bacon when it invented the "Egg McMuffin" (a sandwich). Cousineau is going to shoot up the price more. That (salary for a rookie linebacker) is a lot of Egg McMuffins." CAPTION: Picture, Tom Cousineau