Indianapolis Colts quarterback Andrew Luck announced his retirement from the NFL Saturday night, bringing an abrupt and shocking end to his once-promising career.

"I am going to retire," Luck told reporters following the Colts' 27-17 preseason loss to the Chicago Bears. "This is not an easy decision. It's the hardest decision of my life. But it is the right decision for me."

ESPN reported that Luck, who turns 30 next month, had informed Colts owner Jim Irsay of his decision and would make his formal retirement announcement at a press conference Sunday. But the news broke during the fourth quarter of the Colts-Bears game, prompting a buzz in the crowd at Indianapolis' Lucas Oil Stadium.

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Luck was seen in street clothes on the sideline and addressed reporters after the game.

Earlier this month, Colts general manager Chris Ballard announced that Luck would miss the entire preseason due to lingering pain in his lower left leg. Ballard said tests showed that Luck appeared to be suffering from an injury near the front of his ankle, something his previously diagnosed strained left calf might have exacerbated.

“Honestly, I think it’s a cumulative thing through the years. Andrew’s had some issues with his left [a]nkle] for a while, so I think it’s really a cumulative thing,” Ballard told local reporters on a conference call Aug. 13. “We think we’ve found it. You hope you’ve found what the cause is. Is there a 100 percent guarantee? No, but we think we have, so we’ll move forward and try to treat it.”

Indianapolis Colts quarterback Andrew Luck (12) throws during practice at the NFL team's football training camp in Westfield, Ind., Sunday, July 28, 2019. (Associated Press)

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 Aug. 12, 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 the following day, Ballard said new tests determined the soreness was coming from the front of the ankle.

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The injury had kept Luck out of all of the team’s offseason workouts and all but three training camp practices, and put Luck's status for the Colts' Sept. 8 season opener against the Los Angeles Chargers in doubt. In a recent interview, Luck said he felt as if something was tugging at his lower left leg near the ankle when he ran.

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Drafted with the first overall pick in the 2012 NFL Draft, Luck quickly established himself as one of the best young quarterbacks in the league. In 2014, he threw for 4,761 yards and 40 touchdowns in leading the Colts to an 11-5 record and a berth in the AFC Championship Game. The following season, Luck missed the Colts' final nine games after suffering a lacerated kidney and a partially torn abdominal muscle.

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Luck appeared to return to form in 2016, throwing for 4,240 yards and 31 touchdowns and playing in 15 of 16 games. Shortly after that season, however, Luck underwent surgery to repair an issue with his throwing shoulder and missed the entire 2017 season.

Luck returned last season and led the Colts to their first playoff appearance in four years, won the league’s Comeback Player of the Year award and went to his fourth Pro Bowl.

The Associated Press contributed to this report.