The singer and 'American Idol' judge also dished to ET about his friendship with Underwood's husband, Mike Fisher.
ET's Lauren Zima caught up with the American Idol judge backstage during Day 2 of the 2018 iHeartRadio Music Festival at the T-Mobile Arena in Las Vegas on Saturday, and was the first person to tell Bryan about the singer's exciting news -- which Underwood first announced back in early August.
"Oh that's news to me! That she's pregnant," Bryan said, after being asked about watching the country star perform at the festival. "I'm very, very happy for them!"
It's a bit surprising that Bryan didn't know, considering he said he's "become pretty good buddies" with Underwood's husband, Mike Fisher, as they're both "Nashville guys."
"He has his the guy that built his lake, and my fish guy, we're all kinds of linked," Bryan said. "Mike's building a pond, I built a pond, that's our big connection… because we have little boys, and little boys have to have somewhere to go fishing."
Fisher and Underwood are already parents to a 3-year-old son, Isaiah, while Bryan and his wife, Caroline Boyer, share two sons -- 10-year-old Thomas and 8-year-old Tatum.
As for a baby shower present, Bryan already has an idea for what to get them.
"Well I'm gonna send them some fishing rods," Bryan revealed, adding that he predicts the couple are going to have another boy. "That's so good, I'm so glad to hear that," he added, still marveling at the pregnancy news.
The country crooner also dished on the upcoming season of American Idol, explaining that he is "just looking forward to auditions."
"We've got season one behind us, so we know each other's chemistry now," Bryan said, referring to his fellow Idol judges Katy Perry and Lionel Richie, as well as host Ryan Seacrest. "We all know each other as judges and we're working hard to find the talent and be great judges."
Fans can check out all the performances from the 2018 iHeartRadio Music Festival when it airs Oct. 7 and 8 on The CW.
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