The NYPD sergeant who was slugged by an enraged NFL linebacker in Queens on Saturday was already sidelined for an injury when the gridironer took a cheap shot at him, police sources said.

Sgt. James O’Brien was on limited duty at the 115th Precinct station house in Queens following a recent shoulder surgery when 247-lb Detroit Lions linebacker Trevor Bates flew off the handle and slugged him in the face.

Bates, 25, had been picked up on his way to LaGuardia Airport at 3 a.m., because he had refused to pay a cab fare from Manhattan to East Elmhurst.

O’Brien suffered a black left eye and needed three stitches to close a wound on his brow.

Bates, who is currently being held for observation at Bellevue Hospital, is being charged with second degree assault, obstructing government administration, theft of services and resisting arrest. He faces up to 7 years behind bars if convicted.