Ben Affleck and Matt Damon transformed into Phil Knight and Sonny Vaccaro for the first time on Monday ... and the moment was apparently so big, Jennifer Lopez had to pop by to show her support.
The A-list actors officially kicked off the first day of filming for their new Vaccaro biopic this week in Burbank, Calif. ... with both donning costumes to get into their roles.
Affleck -- who's playing Knight in the untitled movie -- wore a wig and a button-down shirt, while Damon got into Vaccaro character by poppin' on a polo and some khakis.