At the Giants’ annual kickoff luncheon this past August, Tom Coughlin saluted the fans, whom he called the team’s “twelfth man.”

“What I would like,” Coughlin said, “is for our stadium to become the most difficult place in the world to play the New York Giants.” By the time Big Blue get back from Tennessee and their two-game road trip next week, he may find Giants Stadium has become the most difficult place in the world for the New York Giants to play in.

For the second consecutive season, after a promising start, Coughlin’s Giants are unraveling in the second half. And for the second straight season, the reason is the same: after a hopeful start, Eli Manning’s game — his total game — from his mechanics and decision-making to his confidence and ability to inspire confidence — is bad. Right now, for reasons that aren’t clear, Eli Manning is one of the worst quarterbacks in the league.

For the past two weeks, I’ve been pointing out the statistical decline in Eli’s performance. Let’s do an update from the first four games:

Actually, given the support Manning has had from his running game and pass blocking — before Monday night’s 26–10 loss to Jacksonville, Tiki Barber was leading the league in rushing, and Eli had been sacked just three times in his last 76 attempts — Manning might just be the worst passer in the league. He certainly looked like it against the Jaguars. He couldn’t produce a first down until four minutes to go in a first half in which the Giants produced just 73 total yards and converted just one of six third downs.

What exactly is the problem? Coughlin could offer no answers in the postgame press conference, promising only that, “Whatever needs to be done, we’ll make the adjustments. We’ll make them before we get to practice. Then it’s full speed ahead.” But is full speed ahead possible when you’re taking on water? Here are a few things that a review of the game revealed about their passing problems:

• Eli’s play-faking is terrible. Or, rather, he really isn’t faking at all. A linebacker 30 feet away can see that he’s not going to hand a ball off to a running back; in truth, he looks as if he’s extending his hand to drop candy into a trick-or-treat bag. The Giants waste a valuable split-second on third-and-long (of which there are so many these days) by play-faking in those situations. If it’s third-and-twelve, smart pass rushers are going to ignore the running back anyway and go straight for the passer.

• Eli’s vision. From time to time, fundamentals need to be re-taught. For a quarterback, one of those is the art of “looking off” a pass defender. For two straight seasons, I’ve noticed that in the earlier part of the season Manning was quite good at eyeing a number of potential receivers before lasering in on the intended. For some reason, as the season goes on, he tends to stare at the man he is going to throw to practically from the moment he leaves the huddle. Which brings us to …

• Plaxico Burress. When Manning threw his second interception — the final nail in the Giants’ coffin — against the Jaguars on Monday, it was the 12th time in the game that he was gunning for Burress. Meanwhile, he had thrown just 17 other passes to all the other eligible receivers. In contrast, the absurdly underused Jeremy Shockey was thrown to just 11 times and caught seven balls.

In truth, Burress hasn’t come close to justifying the enormous faith the Giants have had in him. Against Jacksonville, he failed to come up with two huge catches that would have resulted in 80 yards’ worth of turf for the Giants. Personally, I thought he came up with both of them even after they re-ran the plays from 12 different angles, but he didn’t do a good enough job to convince the refs. If he had caught both those passes, by the way, Manning’s numbers — 19 of 41 for 230 yards — would have looked a lot better at 21 of 41 for 310 yards.

For two seasons the Giants have failed to develop another key receiver or even work Tiki Barber into a greater role in the passing game. (He caught just one pass against the Jags for 13 yards). If the Giants are ever going to have a consistent passing attack, they have to develop another key receiver.

• The Giants’ pass patterns. Whether the timing is the fault of Eli, or his receivers, or both — and when this happens this often, it’s typically both — the Giants seem to have lost their ability to get the ball to pass catchers while they’re still moving. Far too many of the pass plays result in low percentage throws to receivers who have stopped dead in the middle of coverage. This is the primary reason, I think, that Manning’s pass completion percentage has dropped nearly 18% over the last six games.

Smart coaches know that the best way to build a young quarterback’s confidence is to set him up early with a couple of low risk plays, if only to get a rhythm going. With a tight end like Shockey and running back like Barber, the Giants would seem to be ideally suited for this. But instead, on nearly every down, we’re seeing a blizzard of balls thrown downfield to Burress in double coverage. There’s no use asking whether this is the fault of the quarterback, the coach, or the offensive coordinator when none of them is doing anything to correct it. No matter whose fault it is, we’re seeing the rapid descent of the reputation — and possibly career — of a quarterback who, just a couple of short months ago, was regarded as possibly the most likely young superstar in the league.

Mr. Barra is the author of “The Last Coach: A Life of Paul ‘Bear’ Bryant.”