Corey Feldman, who had previously described the Leaving Neverland documentary as "one-sided", made the U-turn after hearing the claims of sexual abuse made in the film.
In the programme, Wade Robson and James Safechuck alleged they were molested by the singer when they were children - and described the incidents in graphic detail.
The second part of Leaving Neverland was broadcast in the UK on Thursday after initially airing in the US - attracting a mixed response from fans and critics of Jackson.
Feldman, who starred in The Goonies and Gremlins back in the 1980s, told CNN Headline News: "It comes to a point where as an advocate for victims, it becomes impossible for me to remain virtuous and not at least consider what's being said and not listen to what the victims are saying - this is very important.
"I don't want to be perceived as I'm here to defend Michael Jackson, because I can no longer do that. But at the same time, I'm also not here to judge him, because again, he did not do those things to me and that was not my experience."
Jackson's daughter Paris has also defended her father on Twitter, referring to his "good heart" and urging her 1.3 million followers to "think about the bigger picture".
The singer's estate has dismissed the allegations and denounced HBO's decision to air them, claiming the programme spreads falsehoods about a man who is not alive to defend himself.
In response to the claims, The Simpsons has pulled a 1991 episode featuring a cameo appearance by Jackson.
Producer James L Brooks told The Wall Street Journal: "It feels clearly the only choice to make. This was a treasured episode.
"There are a lot of great memories we have wrapped up in that one, and this certainly doesn't allow them to remain."