Irsay told SiriusXM NFL Radio on Monday that Luck's injury is similar to that of former Colts defensive end Raheem Brock.
"I know everyone's had their questions about Andrew and that sort of thing, but I really feel very confident that he's going to find his way through this thing," Irsay said. "I think after the [Kevin] Durant thing [that] everyone's erring on the side of caution, but quite frankly this is not even in the Achilles tendon. It's in another area. It's a bone. You know I'm not good at these things. ... It's a small little bone."
Durant ruptured his right Achilles tendon during Game 5 of the NBA Finals in June.
Irsay did not indicate whether Luck's diagnosis had any effect on his eventual return to the field.
The Colts announced July 29 that Luck was going to stop practicing until he felt like his calf was 100 percent healthy. Luck said the following day that his Achilles was not at extra risk of injury.
The quarterback was ruled out of practices through Monday due to ongoing pain in his calf. The Colts have Tuesday off before starting two days of joint practices with the Cleveland Browns on Wednesday and Thursday.
Luck, who has been a participant in walk-throughs and has worked with throwing coach Tom House, will not participate in those joint sessions with the Browns.
He originally suffered the calf injury in late April. He missed all of the Colts' organized team activities and minicamp in the spring and has taken part in just three practices -- none as a full participant -- in training camp.
Last week, coach Frank Reich didn't shut the door on keeping a third quarterback on the roster heading into the regular season. The Colts play at the Los Angeles Chargers in Week 1.