Doctors have determined that former New York Giants running back Doug Kotar, who remained in guarded condition in a hospital, has a malignant brain tumor.

"The pathology specimen report shows a malignant tumor," said Jenita McDaniel, an assistant administrator at the hospital of the University of Medicine and Dentistry of New Jersey. McDaniel said probable treatment will be some type of radiation.

Quarterback Ken Stabler filed a $20 million dollar libel suit against the New York Times Co. and the National Broadcasting Corp. for reports linking him to a New Jersey gambling figure.

The lawsuit, filed in Houston federal court, claimed the newspaper libeled Stabler Aug. 30, 1981, when it printed a story headlined "Pro Football's Ken Stabler Is Linked to Gambler." The lawsuit said the article suggested a "wrong and malevolent connection with a known gambler," Nick Dudich of Perth Amboy, N.J. . . .

Federal agents questioned an undisclosed number of Dallas Cowboys players about their alleged association with some of the 34 arrested in a series of cocaine raids three weeks ago, a Dallas newspaper reported.

In a copyright article, The Dallas Times Herald reported sources from two federal agencies said agents questioned some players late last week about possible purchases and use of cocaine.

Two players named by federal agents in connection with the investigation are running back Tony Dorsett and defensive end Harvey Martin, the newspaper said. Both Dorsett and Martin denied being questioned by federal agents.