The former Auburn star tore the anterior cruciate ligament of his left knee in Thursday's preseason opener and will require surgery. The Bengals said they expect Irons to recover fully and rejoin the team for next spring's offseason program.
Irons, 23, limped off the field following a 7-yard gain against the Detroit Lions on Thursday night. It was his fourth rushing attempt of the contest and, in all, he gained 16 yards on the ground. The 49th player chosen overall in this year's draft, Irons had been one of the Bengals' standouts in training camp.
In last week's intrasquad scrimmage, Irons carried 10 times for 64 yards.
The loss of Irons means that the Bengals, who were attempting to upgrade at the No. 2 tailback spot, will again use versatile sixth-year veteran Kenny Watson in that role. Chris Perry, the club's first-round choice in 2004, has not been in camp as he recovers from offseason ankle surgery.
The multi-talented Perry, whose career has been marked by a series of injuries, will likely start the season on the NFL physically unable to perform list for a second straight year. By definition, that means Perry will miss at least the first six weeks of the season.
Irons was expected to add speed and big-play potential to the backfield. He was timed at 4.45 seconds in the 40-yard dash at the combine workouts in February and was one of the fastest backs in the 2007 draft class.
After starting his college career at South Carolina, he transferred to Auburn in 2004. Irons finished with 496 carries for 2,366 yards and 18 touchdowns.
Len Pasquarelli is a senior NFL writer for ESPN.com.