The job will go to secondary coach Steve Wilks unless Wilks gets the head-coaching job with the Los Angeles Rams, multiple league sources told ESPN.com.
One of Wilks' first duties would be helping Panthers coach Ron Rivera replace linebackers coach Al Holcomb. A league source told ESPN's Adam Schefter that McDermott will make Holcomb his defensive coordinator in Buffalo.
Wilks, 47, interviewed for the Rams' vacant head-coaching position last week and was granted permission to interview for the defensive coordinator post with the Washington Redskins.
The Washington interview was put off when it became apparent McDermott was headed to Buffalo.
Wilks, a native of Charlotte, joined the Panthers as the secondary coach in 2012. He was instrumental in helping cornerback Josh Norman go from a fifth-round pick out of Coastal Carolina in 2012 to a Pro Bowl selection in 2015.
In December, Norman -- who refers to his alter ego as "Batman" -- credited Wilks for his success.
"Wilks pretty much taught me the importance of using your tools," said Norman, who signed with Washington after Carolina rescinded his franchise tag in April. "Pretty much gave me my tool belt.
"When you look at Batman and his whole little belt, his Batarang he's got around his little stomach, Wilks gave me my first tool belt, so I've got to give him credit for that."
While McDermott gets the credit for having top-10 defenses from 2012-15, Wilks should be credited with having to retool the secondary on a yearly basis because of personnel losses.
He has begun the past four seasons with at least three new starters out of four. No challenge was bigger than in 2016, when the Panthers started rookie corners James Bradberry and Daryl Worley for most of the season.
They struggled early, but by the end of the season Bradberry had a higher overall grade than Norman, according to Pro Football Focus.
"He was really good for Josh," Rivera said of Wilks before Carolina faced Washington in December. "He's been really good for the young corners that we have. He's done a nice job with guys like Captain Munnerlyn and Mike Mitchell. These are guys that came in and he [helped] develop."
Wilks, the secondary coach at San Diego when Rivera was the defensive coordinator in 2009-10, had assistant head coach added to his title in 2015.
He ran the team when Rivera left for two days at the start of the 2015 training camp to attend his brother's funeral. Rivera recently said Wilks was ready to be a defensive coordinator.
"There's a style to his coaching that I think the players respond to very well," Rivera said. "That's why over the past few years we've been able to piecemeal our secondary, because of the way he handles his guys."