ESPN's Adam Schefter first reported Luck was retiring due to mental fatigue.
According to Ian Rapoport of NFL Network, the franchise was aware the 29-year-old was contemplating retirement:
Selected out of Stanford in 2012, Luck quickly established himself as one of the league's top passers. He took over a team that went 2-14 and led it to three consecutive postseason berths, reaching the AFC Championship Game after the 2014 season.
Unfortunately for him, the 6'4", 240-pound star's body took a pounding early in his career. He was sacked 100 times through his first three seasons—and those hits took their toll.
He returned for the 2018 season and played in all 16 games. Despite not playing in an NFL contest in 18 months, he showed minimal rust. He set career highs in completion percentage (67.3) and yards (4,593) while coming within one touchdown (39) of tying his previous best.
Meanwhile, he led the Colts to a 10-6 record and a wild-card berth. Indianapolis beat the AFC South champion Houston Texans in the first round before it fell to the top-seeded Kansas City Chiefs in the divisional round.
That performance helped him capture the 2018 NFL Comeback Player of the Year award.
Injuries, though, have once again popped up. Luck was held out of spring workouts because of a setback described as a calf strain. That was later reclassified as a high-ankle issue. He had not appeared in any preseason games.
With Luck's retirement, Indianapolis will likely turn to Jacoby Brissett under center. The 2016 third-round pick filled in for Luck in 2017 after being acquired from the New England Patriots a week before the season started. He went 4-11 in 15 starts while completing 58.8 percent of his passes for 3,098 yards, 13 touchdowns and seven interceptions.
The 26-year-old has made 17 starts in his three NFL seasons.