Colts quarterback Andrew Luck missed nine games this season with a number of injuries - two with a shoulder injury and then seven with a kidney injury. In addition to that, Luck dealt with a rib injury early in the season, something that wasn't listed on the injury report.
The timeline of Luck's rib injury is still uncertain, but it sounds like he was still dealing with it when not on the injury report with his shoulder injury. That prompted the NFL to look into the potential violation, but according to NFL Media's Ian Rapoport, the league found no violation and will not punish the Colts.
In November, FOX Sports' Jay Glazer reported that Luck had played through multiple fractured ribs. Later in the season, the Indianapolis Star's Stephen Holder reported that Luck had played through torn rib cartilage, and the Colts then confirmed that Luck had dealt with a rib injury - something they previously had not.
The NFL confirmed to Stampede Blue in November that they would look into it, and considering the league's emphasis on the injury reports, it seemed like they might enforce the rules with the Colts' potential violation. This is what the league rules say about the injury report:
All players with significant or noteworthy injuries must be listed on the report, even if the player takes all the reps in practice, and even if the team is certain that he will play in the upcoming game. This is especially true of key players and those players whose injuries have been covered extensively by the media.
This policy is of paramount importance in maintaining the integrity of the game.
Of course, with that said, it's very, very uncommon for the league to find a team in violation of the injury report rules and actually punish a team. In all likelihood, the Colts pointed out that Andrew Luck was already on the injury report with another injury and assured the NFL that the rib injury wasn't serious enough to warrant inclusion in addition. Since Luck was listed on the injury report quite a bit, the league likely found that the Colts weren't intentionally violating any rule (if in fact they violated it at all). So it sounds like nothing will come of the injury report situation from earlier this year, so Ryan Grigson is still maintaining the integrity of the game.