FLOWERY BRANCH -- A pregame injury, mental errors and the lack of depth led to the secondary being exploited in the 43-3 loss to the Dallas Cowboys on Sunday, according to Falcons defensive coordinator Dean Pees.

The unit will be the X-factor when the Falcons face rookie quarterback Mac Jones and the New England Patriots at 8:20 p.m. Thursday at Mercedes-Benz Stadium.

Safety Jaylinn Hawkins sustained an ankle injury, and that meant that rookie Avery Williams had to play nickel back in the slot.

“We lost (Hawkins) right before the game started,” Pees said. “So, we kind of had a thing going on there with him, but that didn’t pan out.”

Hawkins did not play. If the Falcons had known that he was going to be out, they could have used Kendall Sheffield, who played nickel as rookie in 2019 and was a projected starter in 2020 in the former regime’s scheme.

Pees didn’t have the option of playing Hawkins and then moving Erik Harris to nickel back, as he did in the previous game against the Saints. Since Isaiah Oliver was lost to a season-ending knee injury, the Falcons have been mixing and matching at nickel back. They have used least five different players.

Richie Grant, Darren Hall, Chris Williamson, Harris and Williams all have played some nickel. Hawkins has been on the injury report with ankle injury this week, while Sheffield has been buried on the bench.

Williams had trouble covering Dallas’ CeeDee Lamb after the Falcons tried to play man-to-man coverage because they were messing up the zone coverages that Pees had planned to play.

“We can’t make mental mistakes,” Pees said. “We had a couple of mental mistakes in that game that hurt us in zone. So, therefore ... (I) tried calling a little bit of man coverage, which really I did not want to call very much of in that game, based on their receiving corps. But all of a sudden, I’m not putting up with the two mental errors that we had in zone coverage that we play every day.”

The Falcons couldn’t match up with Lamb, Amari Cooper, Cedric Wilson and Michael Gallup, and quarterback Dak Prescott completed 24 of 31 passes, a 77.4% completion rate.

“If somebody beats you, they beat you because they are better than you,” Pees said. “Don’t beat yourself. That’s what we did a lot on defense last week. We made too many mental mistakes.”

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Atlanta Falcons schedule and scores

Eagles 32, Falcons 6

Buccaneers 48, Falcons 25

Falcons 17, Giants 14

Washington 34, Falcons 30

Falcons 27, Jets 20

Bye Week

Falcons 30, Dolphins 28

Panthers 19, Falcons 13

Falcons 27, Saints 25

Cowboys 43, Falcons 3

Next four games

Patriots at Falcons, 8:20 p.m. Thursday, Nov. 18

Falcons at Jacksonville, 1 p.m., Sunday, Nov. 28

Tampa Bay at Atlanta, 1 p.m., Sunday, Dec. 5

Atlanta at Carolina, 1 p.m. Sunday, Dec. 12