Frank Grillo's standing up for Sylvester Stallone ... saying Sly never insulted any extras on the "Tulsa King" set -- and the casting supervisor's lying because she didn't do her job.

We caught up with Grillo -- who's starring in 'TK' season 2 -- at LAX and he says he was on set the day Stallone allegedly launched a string of insults at the extras -- he claims it never happened.

Frank says he thinks casting sup., Rose Locke, got caught not doing her job properly ... adding we're living in a time where anyone can claim their feelings are hurt, and ya gotta deal with it.

Stallone's a gentleman and an absolute professional, according to Grillo ... and says it's sad to see him forced to answer to these types of allegations.

Grillo says he's also caught up with Sly in recent days ... and, you have to watch the clip to hear his update on how Stallone's processing these claims.

As we reported ... Rose Locke, former casting supervisor for "Tulsa King," took to Facebook to claim Stallone called extras names including "tub of lard' and "fat guy with a cane" -- and demanding pretty young girls be brought in to act around him.

Director and Executive Producer of the Paramount+ series, Craig Zisk, told TMZ these claims were false ... saying they needed to shoot a scene in a hip, young bar -- and the extras didn't meet the criteria.

Zisk says they used the extras before he texted Locke and demanded she send headshots of future extras to him to look them over. After a short back and forth, Zisk claims Locke quit.