Dunn lasted just eight minutes of the 4-2 defeat at home to Bury last weekend before he was carried off following a collision which left him needing 14 stitches around his eye.
Manager Gary Johnson said: "Chris' injury is a nasty one and he probably won't be available for a couple of weeks. He hasn't been able to have the injury fully x-rayed yet because of the swelling."
Steve Collis will make his first start between the posts while key midfielder Michael Jacobs will return to the starting line-up after recovering from illness.
Defender John Johnson is fit after a thigh problem but captain Andy Holt continues to be troubled by his knee injury, and midfielder Ryan Gilligan is sidelined for the rest of the season following surgery on a long-standing ankle problem.
Striker Billy McKay is out for another month after dislocating his elbow. The Cobblers have not won in 15 games and could find themselves in the relegation zone if they lose at Adams Park.
Source: PA
Source: PA