The injury occurred in the second quarter on just his fourth carry in Thursday night's 27-26 preseason opening loss to the Detroit Lions.

Irons, a second-round pick out of Auburn in this year's NFL Draft, was on crutches after the contest, and was told of the severity of the injury after an MRI Friday morning.

He is the third key back to suffer a major injury in the past decade, following Ickey Woods (knee), Ki-Jana Carter (knee), and Chris Perry (ankle), all high draft picks.

Irons had distinguished himself in training camp, and was being counted on to be a major backup to starter Rudi Johnson.

The team said in a release Irons should be ready for workouts next spring.