Completion percentage is often the result of play calling more than QB talent.
Consider Philip Rivers, who had the 7th highest completion percentage in football last year, but the 17th highest passing yards for the season, on the 11th most attempts.
Why? Because he dumps it off for short yardage to his check down back or his TE a LOT.
Now consider Andrew Luck (granted, a rookie) who had the 2nd worst completion percentage in the NFL last year, but threw for 4,300+ yards on even more attempts. He had the 5th most attempts in football.
Why? Because Arians relies on a vertical passing game that is high risk, high reward. They take a LOT of big shots down field in his offense, and while that makes the percentages go down, it equates to some big scoring chances and for Indy last year, a trip to the playoffs on Luck's arm.
Detroit takes a lot of big shots down field too. Stafford's completion percentage will go down, while his passing yards remain high, nearly 5,000 of them for last season. When it works well, it's a trip to the playoffs, ala 2011. When it doesn't because of injured receivers and no running game to speak of, we get a 4-12 season.
The easier the throws, the better the completion percentage... but not necessarily the higher the score. The harder the throws, the lower the completion percentage, but not necessarily the lower the score.
A QB's job is to put points on the board and give his team a chance to win. Stafford does that, and is among the best in the game at it. It's even the reason for people questioning his mechanics. He makes plays when most QB's will throw it away. But his completion percentage will be lower because of it.
At the end of the season, it's a team effort, and everyone has to play hard. The numbers might fall on Matty Nails stat line, but they don't belong to him alone, they belong to his receivers and his coaches too.
