The incident was captured on video, which may be difficult to watch. ABC7 has also blurred the video to protect the identities of the victim and those who were not involved in the attack.
The union said the incident happened Sunday night. On the video, the officer can be seen walking through an aisle of bunks when he is sucker punched by an inmate, who then stomps repeatedly on his head.
"We're just very grateful that he's still here to tell this story," said Anthony McGee, vice president of Teamsters Local 700, which represents the deputy.
The union said the probationary officer, who has worked less than 15 months, was ordered by a supervisor to do the safety check of the dorm alone, which they said is a protocol violation.
"The supervisor simply could have left his office and his seated position, went over to the living unit assignment, and served as backup for our member," McGee said. "They chose not to do that."
The beating may have continued if another inmate had not stepped in. Afterward, the union said the supervisor did not call 911 after the incident. Instead, the union says the officer was ordered to get medical treatment at the jail.
A sheriff's office spokesperson said the inmate involved has been charged and an investigation into any policy violations is underway.
"We are thankful our officer was not severely injured and was able to walk away from this incident. Individuals in custody who attack our staff will be prosecuted," the statement said in part.
The union said the attack could have had other consequences.
"Through this vicious assault, our member actually lost the keys to the living unit, his flashlight, as well as the radio," McGee said.
The keys were recovered, officials said.
Full statement from Cook County Sheriff's Office
We are thankful our officer was not severely injured and was able to walk away from this incident. Individuals in custody who attack our staff will be prosecuted. Sheriff's Office investigators asked for attempted murder charges on this case and have secured charges for aggravated battery to a peace officer, a Class 2 Felony. A Cook County Grand Jury indicted Malik Daniel, 28, on that charge today. The Sheriff's Office is investigating if any internal policies were violated.