Affleck, a supporting player in "Ocean's 11" and its sequels, earned raves recently for his performance in "The Assassination of Jesse James."
Affleck's work in his big brother Ben's directorial debut, "Gone Baby Gone," is also generating great buzz.
"Nobody who knows him is surprised," Phoenix said Sunday, while promoting his new film, "Reservation Road."
"(Affleck's) work in 'Assassination of Jesse James' is UNbelievable. It was inspiring and exciting," Phoenix said. "I worked with him 12, 13 years ago. We became friends, he married my sister. It's a really cool story. It's amazing."
Phoenix also said he greatly admired Casey Affleck's determination.
"He worked and persevered. He was told 'no' a million times instead of being supported ... and he persevered. ... He's had a lot of extra baggage that worked against him," he said, referring to Affleck's famous brother and friends.
Phoenix added Affleck "could do everything that we've done and better."
"I guarantee you he could have killed the 'Bourne' series. He could have done 'Walk the Line' walking backwards."