Backstreet Boys singer Kevin Richardson says he didn’t see Flavor Flav get kicked out of the green room at the NBC tree lighting ceremony in NYC … ’cause he woulda stepped in to help his friend if he did.

Kevin was at LAX on Thursday when a photog asked him about what went down backstage at Wednesday’s tree lighting event at Rockefeller Center … including Flav’s claim that security guards kicked him out.

The BSB says he saw FF and dapped him up before their gig, but says the greenroom was a madhouse and security was extra tight in the wake of UnitedHealthcare CEO Brian Thompson‘s murder down the street earlier in the day.

Flav wasn’t on the guest list but managed to sweet talk his way backstage, and when security found out he was there uninvited, they asked him to leave … and Flav made a big fuss about it online.

Kevin says the Backstreet Boys love Flav and would have stuck up for him if they knew he was getting the boot … but he says he didn’t even know Flavor Flav got the heave-ho until after the fact.