We talked to Katie Cherkasky -- who's had years of experience with these sorts of legal matters -- on "TMZ Live" Friday ... where she declared that the now-viral video, in which Diddy can be seen beating Cassie -- should make him and his camp worried.

Katie says the tape gives credibility to Cassie on multiple fronts -- both in the public eye, but also with the feds who are drilling down into major allegations against Diddy right now.

She says the feds aren't going to prosecute him over the crimes depicted in the video -- namely, assault -- but she does say this could potentially serve as a roadmap to other evidence they might be fishing for as the probe continues that might help their case.

Mind you ... Cassie claimed in her lawsuit that one of Diddy's alleged "freak offs" occurred at this same hotel around the same time this incident happened ... so you gotta imagine investigators are eager to dig for more surveillance video that might confirm that accusation.

Of course, as Katie notes here ... prosecutors still have to uncover independent evidence to support the other allegations against him -- which is what they're focused on. Still, she thinks the footage certainly hurts him, as it appears to partially contradict his denial.

TMZ broke the story ... Cassie is cooperating with the feds amid their sex trafficking investigation into Diddy -- which included raids on his Los Angeles and Miami homes.

Diddy maintained his innocence in a social media post last December -- and even on the Cassie front at the time, he was strongly denying all of her allegations ... including this story about him beating her at the hotel. Now, it would seem she was telling the truth about that.