Falcons cornerback Brent Grimes suffered an Achilles tendon injury in the season opener against the Kansas City Chiefs and was placed in the reserve-injured list Monday.
"This morning Brent Grimes had an MRI and it indicated that he has an Achilles tendon injury," said Falcons coach Mike Smith. "He will be placed on the reserve-injured later this afternoon."
Grimes left Sunday's game with what was announced as a calf injury and did not return.
The Falcons were finally getting comfortable with their cornerbacks, but Grimes' injury will throw that position group back into a state of flux.
Against the Chiefs, Asante Samuel started at left cornerback and Grimes moved over to the right side. Grimes played the left side for the past two seasons, while Dunta Robinson was the right cornerback.
Robinson played the slot in the nickel situations.
With Grimes out, Robinson will return to right cornerback, either Dominique Franks or Christopher Owens would move to the nickel back position.
Robinson had a tough day in the slot for the Falcons and Franks and Owens have struggled there in the past.
The loss of Grimes is a major setback to the Falcons' plan to improve their pass defense. The Falcons finished 20th in the league last season as they gave 236.6 yards passing per game.
Kansas City quarterback Matt Cassel completed 21 of 33 passes for 258 yards and one touchdown in the 40-24 victory.
Robinson is more physical in the slot than Franks and Owens, while he has struggled in zone coverage on the outside. Grimes made more plays on the ball outside and will be missed in the team's man-to-man coverages.
"Brent's made a number of plays. Since he's been here, he's been a cornerstone at the corner position, both left and right," Smith said. "It's the next-man-up mentality, and that's how you have to approach it. We've got a number of guys that have play time on our roster right now, and it's something we'll, as a coaching staff, put together what we feel like is the best plan in terms of how we'll configure our nickel package."