Former Bachelors Nick Viall and Ben Higgins also expressed their concern for the new franchise lead.
ET's Lauren Zima spoke with the Bachelor in Paradise alum at the 2018 iHeartRadio Music Festival at T-Mobile Arena in Las Vegas, Nevada, on Friday, where he shared his candid opinion on the new Bachelor franchise lead.
"I love Colton, don't get me wrong, but [he's] not my Bachelor," he revealed. "He's a very nice person... He's well-intentioned. He is beautiful. I just think it's going to be a dumpster fire."
"You know, [he's] a little young. He's younger than me," Unglert expanded, when asked why he thought Underwood was a such a bad choice. "He might be more advanced in his life than I am, but I don't necessarily see him marrying the woman that he chooses... I don't think he knows exactly what he wants."
Unglert himself was once a frontrunner for The Bachelor, until he seemingly shot his chances with an explosive love triangle on Bachelor in Paradise season four. And he seems to see a bit of himself in the franchise's newest selection.
"I imagine Colton's season going the same way mine would [have], where he wants to do right by everyone and wants no one to hate him and that's going to just go ahead and bite him right ahead in the butt," Unglert explained. "I'd be a horrible Bachelor for the same reasons Colton would be horrible and will be horrible."
As for who would have made a better choice? "It should've been Jason, it should've been Blake, it should've been Ben, maybe Nick again. It could've been Peter," Unglert listed.
Underwood was revealed as the new Bachelor on Sept. 4, and like always, Bachelor Nation was divided by the decision. Some fans were pleased to see the wholesome football player-turned-philanthropist get another chance at finding love, while others were concerned that Underwood, fresh off breakups with Bachelorette Becca Kufrin and Bachelor in Paradise star Tia Booth and an emotional reveal that he's a virgin, might not be ready for the job.
And even former Bachelors are worried about the emotional toll that the show could take on Underwood. ET also spoke with Ben Higgins and Nick Viall at the event, who offered their thoughts on the upcoming season.
"I think we're going to see [Colton] struggle," Higgins shared. "I think it's going to be a very authentic journey of like, struggling, success, failure, lot of tears. The problem is, he's pretty indecisive."
"It's very difficult to be the Bachelor and go through that process," Viall agreed. "It's that much more difficult post-filming to try to you know, protect that relationship and kind of go through some of the trials and tribulations that kind of come with the show, the kind of outside influences, and stresses that come with it and again, it'll be curious to see how Colton handles that with limited dating experience in general."
"Everybody takes it very seriously, in terms of their desire to hopefully meet someone and fall in love, because there's a lot of pressure that comes from [being] the Bachelor," he added. "You feel that pressure going through the process, and that part can eat you alive."
However, Bachelor host Chris Harrison has faith that at the very least, we'll get a good season with Underwood.
"No matter what we say, if we said Jason, if we said Blake, it doesn't matter. There's going to be backlash, that's the way social media works these days," he told ET earlier this month. "Colton's going to be good TV, and that is the reason we chose him."
"He probably won't have his shirt on after night one, so it's going to be a good season," he added.
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The Bachelor returns this January on ABC.