When José Mourinho arrived at Tottenham a month ago, his side trailed Chelsea by 12 points. Six league matches later, Spurs will go above Frank Lampard’s side if they beat them at home on Sunday. The turnaround has been remarkable, not only in the way Spurs seem to have rediscovered their edge but in the withering of Chelsea. From apparently clear skies a crisis is approaching and the next two months feel critical for Lampard’s managerial future.

Nobody at Stamford Bridge is grumbling too much yet. Lampard, as one of the club’s greatest heroes, was always going to be indulged more than some grumpy Italian in an ill-fitting tracksuit who chewed cigarette butts on the touchline. He has promoted youth and while, as Mourinho observed on the opening weekend, that can be a handy means of lowering expectations, it has also produced some thrilling football.

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The transfer ban also contributed to the sense that this was somehow not quite a real season, a practice run for everyone. While his time at Derby suggests Lampard would have had faith in youth anyway, it offered a ready-made excuse. He lost Eden Hazard and let David Luiz and Gary Cahill go in the summer and was not able to replace them (albeit the deal for Christian Pulisic had already been agreed); he was operating under an obvious handicap. That restriction, though, has now been lifted, and there is ambitious talk of £150m to spend. This, suddenly, is real.

There remains a profound will for Lampard to succeed. The crowd and the board will be inclined to patience. But as the season draws on, people may begin to remember that Maurizio Sarri, for all he was portrayed as a misfit, did lead Chelsea to the Europa League, the League Cup final and third place in the Premier League. The squad Lampard inherited was not some ragtag bunch of no-hopers.

This sudden dip in results is alarming, although it is probably no more reflective of Chelsea’s wellbeing than the run of seven straight wins (in all competitions) through September and October. One type of failing, though, has remained consistent, and it was true also of Lampard’s Derby. According to Opta, in terms of proportion of shots conceded, Derby were the second-most vulnerable side to the rapid counter in the Championship last season.

This season, Chelsea are the fourth-most vulnerable Premier League side to the rapid counter (Manchester City are, by 0.002%, the third-most vulnerable, which is both unexpected and highly telling of the difficulties they have had with their gegenpressing this season).

Facebook Twitter Pinterest N’Golo Kante (right) has largely papered over the cracks for Chelsea this season but hasn’t been at his best lately. Photograph: Chloe Knott - Danehouse/Getty Images

That could be a particular problem on Sunday given only Leicester have scored more goals on the counter than Spurs in the Premier League this season, while the three players who have blossomed since Mourinho’s arrival – Dele Alli, Lucas Moura and Son Heung-min – all relish driving through space where an opposing midfield should be.

Chelsea’s susceptibility in that regard was apparent on the opening weekend at Old Trafford, when they were probably the better side against Manchester United in terms of general play, yet lost 4-0, shredded by United’s rapid counters.

If there has been an improvement since then it has been largely down to N’Golo Kanté, whose positional sense and prodigious energy mean he can paper over at least some of the structural defects. After four years of supreme consistency, though, he has not quite been at his extraordinary best in recent weeks – which may or may not be related to a continuing legal battle in France with one of his former representatives.

But it is not just counterattacks that are of concern: Chelsea are also unusually exposed at set plays. The proportion of the shots they concede from set plays is 14% higher than the average for the league as a whole (the proportion of the shots they concede from rapid counters is 26% higher than the league average).

It is possible, of course, that Chelsea are exceptionally good at defending from open play. More likely, though, is that they fall down at the two aspects of defending that require rigorous organisation and regular drilling.

That is a particularly galling fault, because it means Chelsea will often not get the results the rest of their game deserves.

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Much of their attacking play this season has been joyous – so good, in fact, that even with 31 goals from 17 games, they are underperfoming their expected goals by four. But that is undermined by leaking soft goals. It undid them most obviously at United, but also at City when they bossed periods of the game, and also at home to Valencia in the Champions League when they lost 1-0 in a game in which the away side barely offered a threat.

Perhaps that is no more than you would expect from a relatively inexperienced young manager, but the worry must be how closely the flaw correlates with the player Lampard was. It is true that an array of managers, from George Graham to Kenny Dalglish, John Beck to Valeriy Lobanovskyi, became successful by rejecting the players they had been. Lampard must be aware of the problem – indeed, it may be that Chelsea’s recent loss of fluency (four goals from their last five games after 27 in 12) stems from an attempt to tackle it – but then everybody was aware of the Gerrard-Lampard conundrum, the inability of two of the best midfielders of their generation to play together, and nobody was able to solve it.

Identifying problems of organisation is easy; actually being able to do the organising is what sets managers such as Jürgen Klopp and Pep Guardiola above the rest – and even the latter is having problems this season.

And that is why the next few weeks feel so vital. Assuming there are new signings, one of the mitigating factors will be gone, but it doesn’t really matter who Chelsea sign if they remain so exposed to counterattacks and at set plays. For Lampard, the honeymoon is drawing to a close. This is when we find out how good a manager he really is.