Harrison will be a guest on the Wednesday episode of On With Mario Lopez, and in a clip of his segment obtained by Us Weekly, he admitted Viall wasn't afraid to be vulnerable on-camera.
"I will say -- a little inside baseball because we're in the middle of taping right now -- he hit a little low point, as all of the Bachelors do," Harrison said of Viall, according toUs.
"You kind of reach this emotional bottom where you get stripped down. If you know what a Mesnick is, I will let you know that he pulled a full Mesnick. There will be man tears on this season for sure."
However, on Mesnick's After the Final Rose special in March 2009, he changed his mind, officially broke up with Rycroft and begged Malaney to take him back. Mesnick and Malaney are now married with two children.
Viall was reportedly enjoying a smooth ride on The Bachelor until a recent turning point when the season became an "emotional mess."
"Nick was handling everything really well -- then everything hit him, and he had a full meltdown," a source told Us. "All of the girls have been crying too. Multiple girls have been crying."
Since Viall, a 36-year-old technology salesman from Milwaukee, WI, was a polarizing choice for the leading role, Harrison vouched for him and also asked fans to have a little faith in The Bachelor producers.
"I know people were a little reluctant, and they're giving him a hard time," Harrison said. "I hope those people will just trust that we know what we've been doing over 15 years. He's a good man -- he's a really good guy. And he's doing a good job."
Many fans are skeptical of Viall since he's already had a shot at finding love on The Bachelor franchise three times, all of which ended with him single or heartbroken.
The Bachelor's 21st season, which began filming in late September, is set to premiere on Monday, January 2, 2017 at 8PM ET/PT.