The Indianapolis Colts believe lingering pain in Andrew Luck’s lower left leg will likely force their starting quarterback to miss the rest of the preseason.

They’re still unsure about his availability for the Sept. 8 season opener against the Los Angeles Chargers.

Colts general manager Chris Ballard said Tuesday additional testing revealed Luck appears to be suffering from an injury near the front of his ankle, something his previously diagnosed strained left calf might have exacerbated.

The injury kept Luck out of all of the team’s offseason workouts and all but three training camp practices. He still has not participated in full team drills.

Ballard and coach Frank Reich initially described the injury as a strained calf, but when Luck continued to complain about soreness, team officials started digging deeper to get answers.

On Monday, Colts owner Jim Irsay told SiriusXM radio the team believed Luck had an injury to the os trigonum, a small, accessory bone near the back of the ankle.

But new tests, Ballard said, determined the soreness was coming from the front of the ankle.

Luck did participate in individual work and some seven-on-seven drills during the first week of camp, but has been limited to working on his own and running walkthroughs since July 29.

Without Luck, Jacoby Brissett has been working with a starting offense that has struggled. Indy’s defense has outplayed the offense consistently at camp and in Thursday’s loss at Buffalo, the Colts generated only two first downs on their first seven series.