Appearing on SiriusXM NFL Radio (h/t The Athletic's Stephen Holder), Irsay said Luck's issue is with a "small little bone" in his foot.
Irsay didn't provide any specifics on the injury, though he did note former Colts players Ryan Diem and Raheem Brock dealt with the same problem at some point in their careers.
Luck's injury has been described as a strained calf he originally suffered in April, which forced him to sit out of Colts organized team activities.
“Just a little tweak; little strain,” head coach Frank Reich told reporters in May. “So my whole mode—and our whole mode—is going to be during this session just to be real conservative with anybody who has any issue whatsoever."
Luck has dealt with significant injuries throughout his NFL career. He missed nine games in 2015 due to shoulder and kidney/abdominal issues and the entire 2017 season recovering from shoulder surgery.
Last season was the first time Luck played all 16 games since 2014. The 29-year-old threw for 4,593 yards and 39 touchdowns.