Director Todd Phillips says Gibson has been replaced by actor Liam Neeson.
Neeson said he got a call asking if he'd like to do the one-day shoot playing a tattooist in Thailand. He accepted the job. He then added that he laughed his leg off when he watched the original.
The film is set to reunite Bradley Cooper, Zach Galifianakis, Justin Bartha and Ed Helms, who starred in the 2009 blockbuster. The sequel is scheduled for release next year.
Meantime, Gibson has been engaged in a high-profile custody battle with ex-girlfriend Oksana Grigorieva.
Authorities are also investigating claims by Grigorieva that the 54-year-old actor-director physically abused her. Audio purportedly of Gibson making a series of racist and misogynistic rants was leaked earlier this year.
A judge has ruled that detectives will have unrestricted access to a computer used by Grigorieva while authorities investigate the actor's claims of extortion.
The Associated Press contributed to this report.