Fans of "The Biggest Loser" have only one more season to watch Bob Harper and Jillian Michaels put the contestants on the ranch through their paces. Jillian has announced that she is leaving for personal reasons. That said, the season has a twist, introducing two new trainers into the mix, possibly so they can fill the void when Jillian exits the show.

"Jillian and I started this whole journey together and we have definitely had our highs and our lows," Bob tells ET. "We will all greatly miss her, but the show must go on. What I will miss about her is definitely her passion and her drive."

The twist for the new season is that in the premiere episode, the 11 family and friend teams will get the opportunity to choose whether to work out with Bob and Jillian at the ranch -- or to work out at a secret location with two unknown trainers. If the contestants choose the new trainers, whom they know nothing about, they'll receive four weeks of immunity. If they choose to stay at the ranch, they're at risk of going home after just one week.

"We found two trainers who are super passionate about changing people's lives and, I think, they are amazing at what they do and the results show it. They get people to drop weight," says executive producer JD Roth, who declines to reveal their identities at this point.

JD goes on to say that he is not closing the door on Jillian forever. If she wants to return, he would absolutely like to have her back.

"Jillian brings a very unique form of television," he points out. "I think she is very special individual. Bob and Jill, to me, are 90-plus percent of the reason that the show is successful. It is the work that they put in that made the show successful. The door is always open for Jillian."

Still JD's take on "The Biggest Loser" is that it is bigger than Bob, Jillian or even him. He says, "The brand is now the Weight Watchers for the 21st century. What 'The Biggest Loser" represents is this lifestyle: The idea that no matter how far south your life has gotten, you can turn it around."

The new season of "The Biggest Loser," introducing the mystery trainers, premieres Tuesday, Jan 4 on NBC.

Related stories on EXCLUSIVE: Larry King Calls Final Show 'Surreal'

Housewife Denies Prince Harry Affair