IT is barely three months since Wes Morgan was hoisting the Premier League trophy above his head, but Leicester City’s success is already feeling more like a blip than a deep-rooted changing of the ways.

Leicester have started the current campaign well enough, following up an opening-day defeat to Hull City with a draw with Arsenal and a win over Swansea, and the gloomier pre-season predictions of a potential battle against relegation look to be well wide of the mark.

Leicester could even find themselves involved in another title race, but as the Premier League returns after the opening international break, there is a sense that normal service has rapidly been resumed.

Manchester City, Chelsea and Manchester United stand at the top of the table with their 100 per cent records, separated by only one goal. That two of them meet tomorrow is a mouth-watering prospect, even at this embryonic stage of the campaign.

Four matches in, it would be nonsensical to describe the Manchester derby as a title decider. But in terms of establishing the mood music for the first half of the season, tomorrow’s lunch-time kick-off at Old Trafford will go a long way towards determining how the next two or three months shape up.

Will it be Jose Mourinho’s preening cockiness setting the agenda by the end of the weekend, or Pep Guardiola’s more measured assurance? The Premier League always touts itself as the ‘biggest league in the world’ – this weekend, and with respect to Celtic and Rangers, who are about to embark on their own midday showdown, it is undeniably staging the biggest game.

For all their prestige and history, you wouldn’t have awarded Manchester United a seat at the very top table when Louis van Gaal was in charge last season. It wasn’t just that United were stuttering on the pitch, although their eventual fifth-place finish proves they were, it was also that their swagger and self-confidence had disappeared.

When Sir Alex Ferguson retired, it was almost as though he took Manchester United’s air of invincibility with him. Opposition teams were no longer scared of visiting Old Trafford, and the club suddenly looked incapable of signing genuinely world-class talent. Less Cristiano Ronaldo, more Marouane Fellaini.

Mourinho has changed that at a stroke. Zlatan Ibrahomivic’s very best days might be behind him, and Paul Pogba was unquestionably overpriced at £94.3m, but by bringing both players to Old Trafford, Mourinho has signalled Manchester United’s return to the big time.

Ibrahimovic and Pogba are the biggest names to have moved to the Premier League for many a year, and the fact they are playing alongside each other in a team that also features Wayne Rooney and Anthony Martial says more about United’s renewed ambition than anything David Moyes or van Gaal were able to utter throughout the duration of their reigns.

Mourinho’s man-management might have been questioned during his dying days at Chelsea, but he has always been able to identify a transfer target and persuade his board to find the money needed to pull off the deal.

There was a cold calculation to Manchester United’s summer business that was missing in the scattergun approach adopted by van Gaal, and whereas last season’s squad looked threadbare, the current side is strong in all positions. The bench for the last game against Hull featured Ander Herrera, Henrikh Mkhitaryan, Morgan Schneiderlin and Marcus Rashford. There aren’t many sides in Europe that wouldn’t take that.

Mourinho has built on last season’s defensive solidity, and added some of the attacking flair that van Gaal was never able to muster. As a result, Manchester United start as deserved favourites tomorrow. If they win, their status as genuine title contenders will only harden.

While Mourinho has transformed the entire mood at Manchester United, Guardiola’s early moves at Manchester City have been less dramatic. In part, that reflects the strength of the squad he inherited, with City having finished ahead of their Mancunian rivals last season and qualified for the Champions League.

But it also highlights the character traits that separate Guardiola from Mourinho. The former has always been more considered and nuanced, building for the longer term rather than slashing-and-burning to solve a series of short-term dilemmas.

There were some significant transfer moves at the Etihad Stadium this summer, not least the £47.5m capture of John Stones and the goalkeeping saga that saw Joe Hart jettisoned to Torino as Claudio Bravo arrived in a £15.4m move from Barcelona.  But whereas Mourinho has turned things upside down on the red side of Manchester, Guardiola has adopted a more evolutionary approach in the blue corner of the city.

Of the 14 players involved in City’s last game against West Ham, only Stones and Nolito were not involved last season. That is not to say that Guardiola has not attempted to change things, but having inherited a side with more issues to address than was the case when he took over at Barcelona or Bayern Munich, the Spaniard appears to have accepted that a period of gradual transition is required.

A win tomorrow would help hasten the development of Guardiola’s blueprint, but while the apparent rebirth of Raheem Sterling has been a huge positive in the early weeks of City’s season, the loss of Sergio Aguero represents a major blow.

Aguero is suspended after elbowing Winston Reid, forcing Guardiola to shuffle his pack and name either Kelechi Iheanacho or Nolito up front. It goes without saying that neither is a like-for-like replacement for City’s talismanic Argentinian.

More than any of their likely title rivals, City are hugely reliant on one player - two if you throw in Vincent Kompany, who is also expected to be absent tomorrow. As Guardiola is already discovering, City’s biggest problem has long been getting their biggest players on the pitch.

With Aguero kicking his heels in the stands, City are likely to be fairly conservative tomorrow, especially in the opening stages. That will place a great deal of pressure on Stones, who is still finding his feet following his summer move from Everton, and Bravo, who is set to make his Premier League debut amid the din of Manchester’s derby day.

This is the first time in the Premier League era that both United and City have gone into a derby boasting a 100 per cent record. At least one club will have lost that status by tomorrow afternoon. It might just be September, but the title race starts here…