How could Matt Damon do The Good Shephard when DiCaprio couldn't due to time constraints from "The Departed"?

I read that Leonardo DiCaprio was Robert DeNiro's original choice to star as the lead in "The Good Shephard" but was forced to cast Matt Damon instead since Leonardo DiCaprio had time constraints due to him filming "The Departed" at the time. Matt Damon also starred in "The Departed" with a role just as significant as DiCaprio's. So how did Matt Damon have the time to film "The Good Shephard" when DiCaprio couldn't?