The Hollywood Reporter did not say why Ruffalo quit the project, however, the actor is in mourning after his brother Scott died Monday, reportedly of a self-inflicted gunshot wound to the head after playing Russian roulette.
The entertainment industry trade newspaper said "Tropic Thunder" star Ben Stiller is set to replace Mark Ruffalo in "Greenburg," the plot of which is not being released to the press at this date.
"Doubt" and "Enchanted" actress Amy Adams, who was to co-star with Ruffalo, has also dropped out of the film and talks are being held with several other actresses to replace her, The Hollywood Reporter said.
Baumbach previously wrote and directed "The Squid and the Whale" and "Margot at the Wedding." He also co-wrote "The Life Aquatic with Steve Zissou" with Wes Anderson and penned the script for Anderson's upcoming "The Fantastic Mr. Fox."
Ruffalo's film credits include "Reservation Road," "You Can Count on Me," "Zodiac," "13 Going on 30" and "Eternal Sunshine of the Spotless Mind."