Adrian Grenier -- REGRETS Suggesting Justin Bieber for 'Entourage' Movie

Adrian Grenier tells TMZ ... he wishes he NEVER named Justin Bieber as the ideal cameo for his upcoming "Entourage" movie ... but still insists the dude could be a great actor under the right conditions.

Grenier -- sporting a cool new samurai-style hairdo -- was leaving The Sayers Club in L.A. last night when we asked about recent comments he made to HollywoodLife.com about Bieber ... when he said, "I saw [Justin's] documentary and thought he was so cool. I think he would be great!”

But last night, Adrian pulled a 180 ... telling us, "Why did I even say it? I started something I didn't want to finish ... so, I don't want to talk about it."

Still, Adrian says Justin has the potential to be great on screen ... because under the right circumstance ... "Everybody can act."

Adrian says he also feels Justin's pain when it comes to criticism about his personal life ... and can relate to JB's urge to race Ferraris.