Phillips, a former general manager of the New York Mets, was fired Sunday after the network said his ability to represent ESPN effectively had been "significantly and irreparably damaged" by revelations about his affair with Hundley.
Steve Lefkowitz, a representative for Phillips, said Phillips has entered an inpatient treatment facility "to address his personal issues."
He stopped short of calling it sexual addiction but compared Phillips' problems to those of actor Michael Douglas, who has been treated for sexually compulsive behavior.
Phillips' wife, Marni, has filed for divorce.
Phillips is still living at home, hopes to return to broadcasting after treatment and had made the reservations at the unspecified rehabilitation facility before ESPN fired him, Lefkowitz said Monday.
"Steve Phillips is not a sleazebag. He's a good guy who just fell off the wagon," Lefkowitz said. "Whatever he did wrong, it wasn't under his control or he wouldn't be checking himself in for help. He knows he needs help and wants a life with his family - that is paramount to him above everything else."
Messages were left Monday seeking comment from Phillips' wife. A message also was left for Hundley, and no one answered the door at her Bristol home Monday.
Phillips' acknowledgment last week of his relationship with Hundley was splashed across the New York tabloids for days, embarrassing the Bristol, Conn.-based sports giant.
Phillips' wife told police in Wilton, Conn., that Hundley called her, sent her a letter and visited their home after he broke off the affair. Hundley also contacted Phillips' 16-year-old son through his Facebook account, according to the police report.
Phillips signed a statement to police that he would not press charges.
In 1998, Steve Phillips admitted having sex with a Mets employee, who sued for sexual harassment. That case was settled out of court. The Mets fired Phillips in 2003.
With his voluntary trip to an inpatient treatment facility, Phillips becomes the latest in a growing number of celebrities to enter rehabilitation programs or seek counseling amid scandals about their behavior.
"If you can say, 'I'm getting treated for this,' it can be a way of saying, 'It's not my fault that I did it,"' said Lori Brown, an associate professor of sociology at Meredith College in Raleigh, N.C.
"We as a society are trying to 'medicalize' almost everything, even criminal or immoral behavior or behavior that's socially inept - but being a jerk is being a jerk," Brown said. "For people who are in the rehab field, it can be irritating to know that someone really needs the resources and can't get them, when someone else may use it just to hide away for a while."