The Tribune Co., which last Dec. 18 put its New York Daily News up for sale, today abruptly pulled the tabloid off the sales block and announced that it would seek concessions from the paper's 11 unions to save the 63-year-old paper.

The move came just 48 hours after the Chicago-based publisher terminated all discussions with would-be buyer Joe L. Allbritton, who had been negotiating concessions from the union since April 1.

In the paper's newsroom, members of the emotionally battered staff showed little excitement over the announcement. Only a week ago they had lined up to learn if their names were on the "hit list" of nearly 200 editorial employes Allbritton sought to cut.

"There is no demonstrable joy here because people have been through too much to get excited over a press release," said David Hirshey, editor of the Daily News Sunday magazine.

Tribune Co. President Stanton Cook said the News' "circulation and advertising strength demonstrate it will be needed and effective in the marketplace for years to come."

The News suffered its first loss of $11 million last year, but today Cook said the News could be profitable again, "depending on the amount of cost reductions we can develop with the unions' help."

The Allied Printing Trades Council, an umbrella union group, quickly agreed to negotiate. News Publisher Robert M. Hunt said management will come up with cost-cutting proposals in about 10 days. "They will require sacrifice, but management will share that sacrifice," Hunt said.

Rupert Murdoch, publisher of the rival New York Post, is seeking to keep alive his proposal to buy the News, but today Cook labeled Murdoch's latest offer "patently illusory" and accused the rival of committing "an anticompetitive and predatory act."

Murdoch recently claimed that the New York Post lost $20 million last year and expected the same in 1982.

It is unclear why the Tribune Co. suddenly changed its view of the News' future. Although circulation has been climbing, much of that is apparently due to the success of a bingo-type game published in the newpaper and other costly promotional moves that will be difficult to sustain, News officials say.

Allbritton staff members have been told not to discuss their feelings about the breakdown in negotiations. But some members privately have said they felt that, once the Tribune Co. saw the Allbritton forces making progress with the unions, the Chicagoans decided to keep the paper.

Edward Silver, a New York labor lawyer retained by Allbritton, said the Tribune Co. managers changed course "when they realized they could negotiate savings from the unions."