Fox underwent successful heart surgery last week having felt dizzy and light-headed during a round of golf, raising concerns that the stressful nature of his job had taken its toll.
However, the 58-year-old revealed that he had been monitoring the situation since the problem was first diagnosed in 1997 and was told that he would probably require surgery at the end of the current season.
"[It] was progressively getting worse a year ago; they thought it would last another year," Fox told reporters.
"I'm very, very healthy. It wasn't due to lifestyle, not being healthy, too much stress. It's basically something I was born with. This is really not a lifestyle problem.
"Sure, there's some pressure and stress involved in coaching, but I think a lot of people out there, in Denver, in the country, really around the globe have pressure-packed jobs. I think our military comes to mind as one of those I don't think coaching compares to. It wasn't the pressure of coaching or that kind of thing."
Fox's return date is as yet unknown, but he did express his desire to make it back before the end of the season.