The Chargers dominated possession during the game, enjoying almost twice as much of the ball as their opponents, and restricted the much-fancied Colts offense when they finally did get onto the field.
The Colts were successful on just two of 10 third down attempts on their way to a 19-9 defeat, and Luck believes that the Chargers' defensive performance was the main reason behind their inability to improve those figures.
"We stunk on third down today. A lot of credit to their defence. I think we made our fair share of mistakes, but I think it's what they did," Luck told The AP.
"If we can't convert third downs and drives continually stall like that, it's a short rest for our defence and no points for us, so it's a bad combination."
Despite the result, the Colts are still top of the AFC South but face a tough match against the unbeaten Denver Broncos on Sunday.