A source tells PEOPLE the actor, 47, “has and will always be there for Ben.”
“They have been friends for years,” the insider says, adding, “Let’s remember that they each have their own families and careers to deal with.”
A second source confirms to PEOPLE Damon has been involved in supporting Affleck since he entered rehab on Aug. 22.
Damon is currently filming the movie Ford v. Ferrari in Los Angeles.
The two made it big in Hollywood together after winning Oscars for writing Good Will Hunting in 1998.
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In an interview with Barbara Walters in 2012, Affleck credited Damon as one of the people who helped him get through his battles with addiction.
“I had good friends. I had Matt Damon, I had my brother, and I had a nucleus of good friends that I grew up with and I had support from,” Affleck told Walters.
The two recently won the screen rights to a true crime story about an ex-cop who rigged the bi-annual McDonald’s Monopoly game in 2001. Affleck is to direct Damon in the project, which marks their first film together in over a decade.
Affleck entered rehab last week. A source close to the actor told PEOPLE, “He knew he needed help and was vocal about it.”
By his side was ex Jennifer Garner, also 46, who arrived at his home with an addiction therapist. Garner drove the actor to a Malibu treatment center the same day.
Several sources say Affleck had in fact been struggling again for months, though he had been going to meetings and working with a sober coach for much of the last year.
“He’s battled addiction for a long time now,” says an Affleck source, noting the star had been “constantly working on himself” but after splitting with Lindsay Shookus in July “went into a darker and darker place until he had to reach out for help.”
The actor previously spent time in rehab in 2001 and 2017.
Affleck wrote on Facebook in 2017 after a previous stay in rehab: “I want my kids to know there is no shame in getting help when you need it, and to be a source of strength for anyone out there who needs help but is afraid to take the first step.”
If you or someone you know is in need of help, please contact the SAMHSA substance abuse helpline at 1-800-662-HELP.