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Santana Moss says he has the PERFECT candidate to replace Jay Gruden on the Redskins ... telling TMZ Sports Gregg Williams would KILL IT as headman in D.C.!

Of course, Moss is a bit biased ... he was playing receiver for the 'Skins while Williams was the defensive coordinator there back in the early 2000s.

But, Moss says that's exactly why he's pounding the table for Gregg after Gruden's firing Monday ... saying his time with Williams has convinced him the 61-year-old should be Dan Snyder's next hire.

"I watched our defense for years," Moss says ... "One year, we had a lot of missing bodies out there -- like, key guys. And, he made us a contender."

Moss says Williams' defenses were so inspiring, it actually made the offense play better ... telling us, "He implemented what we needed as a culture."

Williams is currently the defensive coordinator in New York ... but it seems like he might be available for hire in 2020, 'cause the Jets are currently terrible.

And -- even though Williams is seemingly constantly surrounded by controversy (see Bountygate and the 2019 Odell Beckham drama) ... he does have a SERIOUSLY impressive resume.