Three straight seasons with a 11-5 record from 2012 to 2014 betrayed the fact that the Indianapolis Colts had been steadily improving in each season of the Andrew Luck era.

During Luck's rookie year, the Colts had a -30 point differential and were bounced in the first round of the playoffs. The following year, they outscored opponents by 55 points and won a playoff game, and in 2014, they were plus-89 and reached the AFC Championship.

Last season, this trend came to a screeching halt.

Luck missed nine games and struggled when he was on the field, as the Colts went 8-8, missed the playoffs and were outscored by 75 points (tied for the seventh-lowest point differential in the NFL).

Indianapolis finished 24th in our nERD power rankings, which imply they would have been expected to lose to an average team on a neutral field by about five points.

What Went Right

In a season full of surprises, the Colts' run defense was the rare pleasant one.

What had been the team’s weakness for the better part of the past 15 years was actually the team’s strength, as Indianapolis ranked 17th in opponent-adjusted 23rd, allowing 4.3) actually might undersell how successful they were against the run.

Only about 35% of opponents rushes yielded positive expected points against the Colts, compared to the league average of about 40%.

Success Rate is a more reliable measure of rushing efficiency than raw yards per carry, which can be skewed by a small number of large gains.

Only four teams allowed a first down on the ground at a lower rate than the Colts (20.3%), who also tied for eighth in rushing conversion rate allowed on 3rd-and-2 or shorter.

Linebacker D'Qwell Jackson was eighth in the league in tackles for a loss with 19, while rookie lineman Henry Anderson added 13 (in nine games, before suffering a season-ending injury) and Erik Walden had 10.

In terms of run stops on third and fourth downs, only three players had more than linebacker Jerrell Freeman, while Jackson and Walden added four apiece.

The Colts were gashed by long runs, as only San Diego and New Orleans allowed more carries of 20 or more yards than the Colts (who allowed 16), and this dragged down their ranking in rushing average.

That said, rushing success rate tends to be more predictive of future winning than yards per carry, which is good news for the Colts.

In terms of other bright spots from the season, T.Y. Hilton was 13th among wide receivers in Reception NEP, and Donte Moncrief finished 26th. In terms of Reception NEP per target, Moncrief tied for 22nd, while Hilton tied for 31st (minimum 50 targets).

On special teams, Pat McAfee was second in the league in yards per punt, while Adam Vinatieri was fourth in the league with a 92.6% field goal percentage (including 12 makes on 13 attempts from 40 yards or longer). Vinatieri finished fourth in Field Goal NEP (22.71) and was fourth in Field Goal NEP per attempt (0.84), making him one of just four players to top 0.62.

What Went Wrong

Teams that cannot pass or stop the pass are going to struggle in the NFL, and the Colts found that out the hard way.

Only the 49ers and Browns had a lower net yards per pass/net yards allowed per pass ratio than the Colts, who averaged a league-low 5.5 net yards per pass and allowed 6.6.

The Colts were 24th in opponent-adjusted Passing NEP, thanks to Andrew Luck’s struggles and the fact that (contrary to the #QBWinz crowd), Matt Hasselbeck was not any better.

After tying for seventh in Passing NEP per drop back in 2014 with 0.19, Luck dropped to a tie for 19th with 0.08. The fourth-year passer completed 162 of 293 passes for 1,881 yards, 15 touchdowns and 12 interceptions, while also getting sacked 15 times for a loss of 88 yards.

His 4.1% interception rate was unsustainable, but his struggles were hardly just the product of random variation. Luck also averaged a career-low 5.8 net yards per pass and had a 44.5% success rate and 55.2% completion percentage (both the lowest rates since his rookie year).

The Colts' offensive line did him no favors, but Luck had shown an ability to overcome poor protection in the past. He also still ranked 13th in sack rate (4.9%) and was better than the league average of 6.1%.

Luck reportedly played hurt during the year, even before an injury ended his year in early November, and this is not hard to believe, given he had been steadily improving into one of the game’s better quarterbacks over the past three seasons. It would take a pretty sizeable leap of faith to assume his skills all of sudden diminished.

Nevertheless, Luck did not play well, and it played a pretty big role in the Colts’ early struggles.

The Colts did win more with Hasselbeck in the lineup (winning his first four starts and going 5-3 in his starts overall), leading to the silly narrative in certain corners of the internet that the team was better off without its franchise quarterback.

While Hasselbeck, a 40-year-old backup, deserves credit for playing at a close to league average level, his numbers were comparable to Luck’s, and the uptick in winning percentage was largely a function of the Colts’ schedule (Hasselbeck’s average opponent had an average nERD ranking of 19, compared to Luck’s average opponent, who had an average ranking of 12.6).

Hasselbeck completed 156 of 256 passes for 1,690 yards, 9 touchdowns, and 5 interceptions, while taking 16 sacks for a loss of 101 yards.

He did have the better completion (60.9%), success rate (46.7%) and interception rates (2.0%) but also had the same net yard per pass average as Luck and a lower touchdown rate (3.5% to 5.1%). This gave Luck a higher Passing NEP per drop back rate (0.08 to 0.05).

While you could easily make the case Hasselbeck performed well enough given expectations, neither his nor Luck’s performance was good enough, and the Colts' offense (24th in overall opponent-adjusted NEP) suffered as a result.

The Colts' defense was 20th in Adjusted Passing NEP allowed per play, and their biggest problems were big plays through the air.

Like the rush defense, the Indianapolis secondary posted a solid success rate, ranking ninth in completion percentage allowed (60.5%) and also tied for sixth in interception rate (2.9%).

They also tied for most passing plays over 20 yards allowed, knocking the Colts' net yards per pass average to 6.6 (tied for 18th in the league).

Pass rush was part of the problem, as the Colts tied for 22nd in sack rate, but after adjusting for context and opponent, Indianapolis ranked 28th in adjusted sack rate, per Football Outsiders.

What’s Next?

It almost goes without saying, but the 2016 Colts’ fate rides on Andrew Luck’s health and return to form.

Passing is king in the NFL, and as Luck has gone, so too have the Colts.

Improving Luck’s protection will probably be a priority this spring, as will improving the pass rush and secondary.

Freeman, Vinatieri, tight ends Coby Fleener and Dwayne Allen, and corner Greg Toler are the team’s biggest pending free agents.

Toler has been injury prone and has been a culprit on many of the big pass plays that plagued the Colts, so the team could let him walk and search for a new starting corner opposite Vontae Davis.

Vinatieri told the Indianapolis Star he plans on playing in 2016, while things are trickier in terms of Freeman, Fleener and Allen.

Freeman has been a solid starter for the Colts, particularly against the run, but will be entering his age-30 season, so it is hard to know what the team’s plans for him are.

As for Fleener and Allen, in a perfect world, both would be back to keep the Colts' dynamic receiving corps in tact. Allen might be the more complete player and has a better catch rate (61.9%) and yards per target rate (7.1) than Fleener (61.9% to 58.8%, 7.1 to 6.9), and is also considered the better blocker of the two.

Fleener, though, has been a bigger part of the passing game, averaging more targets per game (5.2 to 3.4), and also had a much better 2015.

Last season, Fleener caught 54 passes on 84 targets for 491 yards (64.2% catch rate, 5.8 yards per target, 9.1 yards per catch), and averaged 0.43 Reception NEP per target.

Allen caught 16 passes on 29 targets for 109 yards (55.2% catch rate, 3.8 yards per target, 6.8 yards per catch).

This team has flaws, and it is doubtful they can fix them all this offseason. 

That said, if you can only do one thing well in the NFL, it should be throwing the ball, and if a healthy Andrew Luck can do that, Indianapolis should be back on track in 2016.