Nielsen joined Atlanta to replace Dean Pees, who retired in 2023. The Falcons ranked 11th in yards allowed and 18th in points allowed. However, former head coach Arthur Smith was fired after his third consecutive 7-10 season. The Falcons then allowed the assistants on their staff to interview with other teams.

Nielsen, 44, previously served on New Orleans' coaching staff from 2017-2020. After serving as defensive line coach for the first four seasons, he was promoted to assistant head coach/defensive line coach in 2021. In 2022, he was named co-defensive coordinator/defensive line coach.

Jacksonville fired former defensive coordinator Mike Caldwell after the club ranked 17th in points allowed and 22nd in yards allowed in 2023.