There is no definitive answer to the question of when a slump becomes a descent into decline.
The concern at Stamford Bridge is that, in Chelsea's case, it already has. The statistics can be quoted - two goals, a deflection and a penalty, and two points from four games; one win from six; a five-point lead at the top of the table being turned into a two-point deficit - but the numbers that have always suggested difficulties lay ahead for Chelsea are found not in the sports pages, but on the birth certificates.
Today is John Terry's 30th birthday. It is not merely the recent results that should prevent it from being a day of celebration at Stamford Bridge.
It is the knowledge that it is a milestone Didier Drogba, Frank Lampard, Nicolas Anelka and Florent Malouda have already reached and one which beckons for Ashley Cole later this month. Collectively, it is Chelsea's ticking time bomb. The detonator is being readied.
Drawing against Everton, as Chelsea did on Saturday, is no shame.
Indeed, David Moyes's side procured a point at Goodison Park last December, a month when Carlo Ancelotti's team struggled. That did not lead to decay.
This might. Because the long-term issue of the succession grows ever more pressing with each year that passes. Obscured by the silverware that has gleamed at Chelsea, overlooked among the unprecedented glut of goals they have delivered in the past 12 months, it has nevertheless been a constant theme.
In a career of phenomenal fitness, Lampard has played more than 750 games for club and country but only five this season. The years may be catching up with him.
A patched-up Terry returned on Saturday, but his is the body of a 35-year-old. Malaria has sapped Drogba of his formidable strength and, with his close friend weakened, Malouda has experienced a deterioration in his own game.
The ageing process is compounded by the reality that too few of the younger generation, Michael Essien apart, possess the same presence.
John Obi Mikel and Ramires are cases in point, expensive recruits who do not dominate. It is always left to the senior players to lead. As their numbers decrease, the significance of each one is exacerbated.
In many respects, they have already thwarted the passing of time. Sir Alex Ferguson said earlier this season that teams have a four-year window when they can win silverware before renewal is needed.
Chelsea confounded that particular theory by extending it to six seasons, from Jose Mourinho's arrival in 2004 to the league and cup double that was secured in May.
But transition is awkward to execute without standards slipping.
Ferguson has built a succession of Manchester United teams. Chelsea are yet to prove they can do likewise.
The teenage talents are confined to the bench (and, Josh McEachran apart, there is some doubt as to how good they are) and attempts at regeneration have only consisted of dispensing with some of the veterans. Thus far, Ramires has been an inadequate substitute for Michael Ballack. Deco and Juliano Belletti have not been replaced, which may not have mattered. Crucially, however, Ricardo Carvalho's departure left a void.
While parallels are being drawn with the dark days of the end of Luiz Felipe Scolari's reign, it leaves Chelsea looking as stretched as they were in Mourinho's final full season, 2006/07, when Roman Abramovich decided the squad needed pruning.
Then they rallied and recovered, winning two trophies, reaching a Champions League semi-final and coming second in the league.
It is a tribute to the team Mourinho constructed that they have maintained remarkable standards of consistency over six-and-a-half years. It is the product of an awesome will to win. The spirit is surely still willing, but if the legs are not then even a manager as skilled as Ancelotti is hamstrung. Chelsea have defied predictions of their decline before. They face their biggest challenge yet to do so again.
These are not happy days in west London. Fulham are unfortunate that their achievements tend to attract comparatively little attention.
But so, too, do their failings. Their performance at Arsenal was reasonable, but the reality is that the Europa League finalists have taken two points from five games. Barring a rapid improvement, they are in a relegation struggle and that makes it a sudden fall from grace.
"We hope the game will be on. It would be very embarrassing if we can't get it on." So spoke Ian Holloway last week, less than 48 hours before the first postponement of the Premier League season.
Blackpool's match against Manchester United was billed as the biggest Bloomfield Road would have staged for 39 years.
Instead, on a weekend when every other Premier League game took place, Blackpool, a town where the coastal air makes it appreciably warmer than Blackburn, Manchester, Leeds and Sunderland, to name but four places that staged matches, had no fixture.
To those acquainted with the blend of penny-pinching and rank amateurism at the club, however, it was no surprise. For the deserving manager, players and supporters, it was not just embarrassing Blackpool could not host United. It was insulting.