After making a selection of critically-panned movies - including Gigli with then-fiancee JENNIFER LOPEZ - Affleck's acting skills were questioned by a number of critics.
But Damon, who grew up with Affleck in Massachusetts, is sure his pal will earn acclaim.
He says, "He just did Truth, Justice And The American Way. He's going to be great. People forget that he's a terrific actor, and maybe through some fault of his own, he hasn't made the best choices in the last couple of years.
"When this movie comes out and it's great, everyone will be like, 'It's a comeback.'"