(CNN) -- A 2-year-old son of Minnesota Vikings star Adrian Peterson was in critical condition Friday after allegedly being assaulted in South Dakota, the running back's father told a Minnesota newspaper.

Sioux Falls, South Dakota, police said Robert Joseph Patterson, 27, was arrested on charges of aggravated battery of an infant and aggravated assault.

Patterson originally called authorities at 5:46 p.m. Wednesday to a report a medical emergency involving a 2-year-old child in his Sioux Falls apartment. Police arrived to find that the toddler was unresponsive and later determined that he had suffered injuries consistent with abuse, according to the department.

Patterson -- whom police described as the boyfriend of the injured child's mother -- was arrested the following day, said Lincoln County, South Dakota, State's Attorney Tom Wollman.

As of Friday morning, the 2-year-old was in critical condition at a South Dakota hospital, according to police.

Adrian Peterson's father, Nelson Peterson, told the St. Paul Pioneer Press that the injured child is his grandson.

The NFL standout himself did not elaborate on what happened when addressing reporters Friday, though he did acknowledge that he was dealing with "a private matter."

"I will ask you all to please just respect my privacy," Peterson said.