Which Colt has been the biggest disappointment?

The pillar of the Indianapolis Colts' besieged and beleaguered offensive line stood at his locker late Sunday afternoon, dressing slowly, silently, piecing together a disastrous day in what's becoming a disastrous year. Anthony Castonzo fumed.

"I've been (expletive) terrible this season," the Colts' $42 million left tackle said a moment later. "I'm just going to call it like it is."

Then he did. Castonzo was blunt. He was brutally honest. The frustration poured out of him — his NFL-high five holding penalties this year, his three false starts, his lapses in technique, the mistakes, the mistakes, the mistakes. "Me doing stupid (expletive)," he called it. Castonzo would not skirt the obvious after his team's 27-21 loss, nor would he make excuses, nor would he rationalize his second-rate start to this 2015 campaign.

The Colts are 3-4. He's one of the reasons why.

"I have to play better, straight up," he continued. "Forget what my contract is. As the starting left tackle on this team, my teammates rely on me. I don't care if I was making $5 a game; it doesn't matter, they expect me to get the job done and I'm not doing that."

He's not alone. And that's the problem. These Colts, the team that dished out one fat contract after another in the offseason, the team with the soon-to-be-slipping-away Super Bowl ambitions, the team that couldn't even beat the hapless New Orleans Saints on Sunday, are now little more than a collection of underachievers.

You can't win in this league when your stars are playing like duds.

"We have the potential to be an explosive team, but potential only gets you so far," was how tight end Dwayne Allen aptly summed it up.

Castonzo isn't the only key cog not pulling his weight. It's an epidemic. And it's burying this team.

The franchise quarterback? Andrew Luck's thrown eight interceptions in five starts. The team's 1-4 when he plays.

The 34-year-old receiver the Colts handed a $21 million contract to in March? Andre Johnson's had one standout game all year.

The 33-year-old rush linebacker the Colts handed a $16 million contract to in March? Trent Cole doesn't have a sack all season.

The 33-year-old left guard the team signed to shore up the interior of its offensive line? Todd Herremans was benched after two games.

It's Castonzo, it's Luck — stalwarts of this team, this year and for many more to come — off to the worst starts of their career. It's Johnson, it's Cole, it's Herremans — the talent the Colts lured to town last spring that was supposed to help this group crest the playoff mountain. Problem is they're the ones most guilty for halting the climb.

"They say your identity is what you put on tape," Luck said. Later, he held firm: "We're a darn good football team."

But they're not. Not after digging themselves a 27-point hole Sunday. A darn good football team? No. Not even close. This is a mediocre football team with darn good players playing far beneath their potential.

It's what led to a heated conversation between owner Jim Irsay and General Manager Ryan Grigson in the locker room after the game. It's what led to their desperate coach, Chuck Pagano — whose seat seems to be warming by the minute — giving an impassioned postgame speech to his players. It's what has led to a Super Bowl contender being below .500 seven games into the season.

Coming are showdowns against undefeated Carolina and undefeated Denver. Meanwhile it's nearly Halloween and the Colts are a team without a semblance of an identity. Besides this, of course: They're winless outside of the AFC South.

"As far as frustration towards each other or finger-pointing, there's nothing like that," said Johnson. "We have guys who say we can't dig holes like this. We know we're a good football team. We're just not playing like it."

But there is anger. There is disappointment. There is disbelief.

"If I need to give guys advice, if I need to fight somebody, if I need to give somebody counsel or be a big brother, whatever it is, we've worked too hard," said Robert Mathis. "We have to stay as one."

That's all they can do at this point. Their return on offseason investments has been dreadful — to say nothing of the first-round pick, receiver Phillip Dorsett, who's caught just 10 passes all season and is expected to be sidelined for the next month with an ankle injury. The star players were mostly to blame Sunday. Luck threw two errant interceptions. Castonzo, whistled for both a hold and a false start, was humbled relentlessly by Saints end Cameron Jordan. Johnson caught four passes. Cole had five tackles. Herremans watched in street clothes.

Suffice to say the Colts aren't paying Johnson $7.5 million and Cole $8 million for that. They're not exactly stealing money — not quite — but they're not exactly living up to those lofty contracts.

The frustration inside the locker room has been building for weeks. From the 0-2 start to the fake punt fiasco last Sunday night to this, the low point in a season that figures to have a few more. They've stumbled, over and over, failing still to piece together a complete, 60-minute effort.

Most concerning: No one seems to have an answer as to why.

"We have to be our biggest critic," cornerback Greg Toler said. "Every man has to look himself in the mirror and figure out how they can make more plays."

The left tackle's biggest critic, standing in front of his locker, continued with his candor. Castonzo was speaking about himself. He might as well have been talking about the whole team.

"The bottom line is that just wasn't good enough. Not even close."

That, in 12 words, is the story of this team's season.

Call Star reporter Zak Keefer at (317) 444-6134. Follow him on Twitter: @zkeefer.