It is either Aaron Glenn or Dennis Allen, or both.
The Saints Defensive Line is extremely deep, players improving by huge amounts every season. No question Ryan Nielsen is killing it as the DL coach. Rankins, Onyemata, Hendrickson, and Davenport are all trending upwards in a big way, and they're all home grown drafted players. Cam Jordan's game is just on another level completely since Nielsen showed up.
Mike Nolan only has one home grown player to work with, Alex Anzalone, and the only hole in his game is staying healthy. He improves by leaps every season. Davis, Klein, and Robertson have all also thrived with Nolan at the helm of the Linebackers.
The Secondary is stagnating and regressing. Aaron Glenn is the Secondary Coach. Dennis Allen is the Defensive Coordinator, but his specialty is the Secondary. Ken Crawley was a very highly rated corner in 2017 and looked like the perfect compliment to Lattimore. Ken Crawley is now a healthy inactive. Marshon Lattimore had a rookie season that made you think Revis 2.0. He hasn't come close to that since then. I believe Vonn Bell only missed two tackles his entire rookie season? Now it's no surprise to see him whiff completely. Marcus Williams had an incredible rookie season, but he's been lost every since. Patrick Robinson has been a disaster here. PJ Williams is barely surviving in the slot, and is horrible on the boundary. Eli Apple looks worse this season than he did last season. I understand you can't hit on every draft pick and free agent signing, but the "failure" rate right now among the secondary is alarming. Something's gotta give. No way can the Saints hit on nearly every defensive lineman and linebacker they draft/sign in free agency while the secondary is just miss after miss after miss. This screams coaching to me.