"After he met Amal, yes [I thought he would have children]. I wasn't surprised when he told me. I'm extremely happy for him and for them," Damon said at the Los Angeles premiere of his movie The Great Wall.
"Once he met her, I had a feeling that was in the cards," he added. "It wasn't a question for many years -- it was a total nonstarter [if he would start a family]. Once he met her, everything changed."
The Talk host Julie Chen had announced last week that Clooney and Amal, who married in 2014, are expecting twins in June. Damon himself is the father of four daughters with wife Luciana Barroso: Alexia, Isabella, Gia and Stella.
"[George has] got this group -- a really wonderful group of friends," the actor said on Thursday's episode of The Ellen DeGeneres Show. "All of those guys, their kids are either in college or on their way, and George is just getting started."
"He said to me, 'Well, you have four.' And I said, 'Yeah, but not at once. We did it one at a time.' I don't know how anybody does twins. Those sleepless nights, and then there's two of them? There's just no way. But we'll see what happens."