The Oscar winner confessed that while Clooney, 55, is often overly prepared on set, he is still prepping for this whole first-time fatherhood thing. The news broke in early February that Clooney and his wife, Amal, are expecting twins in June.
"He's going to get prepared," the Suburbicon star told ET. "He better figure it out, 'cause they're on the way. But he'll be great. He'll be a mess, but [Amal] will take care of everything."
Damon, who stars alongside Julianne Moore in the crime comedy, said he always has a blast working with the Money Monster star. "I'd do the phone book with George," he admitted. "He's just a great, great director. He makes it a lot of fun, so hopefully he'll have another gig for me."
As for his longtime friend, Ben Affleck, Damon says he's "fantastic," telling ET, "He's with the kids now, couldn't be happier. Jen's working for a couple weeks in Atlanta, so he's Mr. Mom-ing it right now. And that's what he wants to be doing."
Affleck, 44, recently revealed that he completed rehab for alcohol addiction in order "to be the best father I can be" to the three children he shares with estranged wife Jennifer Garner -- Violet, 11, Seraphina, 8, and Sam, 5.
Damon added that he is "lucky" to be surrounded by such a solid group of guys. "I have a good group of friends," he said. "We always keep track of each other and look out for each other."
Watch the video below for more on Damon's friendship with Affleck.