Can Liverpool succeed where everyone else has failed and beat Man City?
It is the dilemma that has confronted every Premier League manager: How do you stop the seemingly unstoppable?
Manchester City are unbeaten in 33 games over almost nine months against domestic opponents. They have dropped points against only two teams this season, and on one of those occasions, they played the majority of the match with 10 men. They have scored almost five times as many league goals as they have conceded and could be crowned champions in March.
English football's most unsolvable problem, stopping Man City, has brought a range of answers from the original to the derivative, the typical to the out-of-character, the partial successes to the outright failures, resulting in the damage-limitation exercises to the damaging defeats. The chances are that when it is his turn on Sunday, Jurgen Klopp will use the 4-3-3 formation that worked at Anfield last season and perhaps, but for Sadio Mane's first-half red card, might have succeeded at the Etihad Stadium.
September's game against City ended in a 5-0 defeat for Liverpool after Klopp changed to a 3-5-1 formation and strangely substituted Mohamed Salah at the interval. The evidence of this season is that quick, skillful players are as likely to trouble City as anyone.
The common denominators among the better efforts involve counter-attacking, congesting the midfield with defensive-minded, tactically disciplined players and focusing on set-pieces that, in the span of eight days, brought Huddersfield and West Ham goals and almost produced another for Southampton. The more obdurate opponents have combined defensive resolve with some attacking intent: Bristol City, who defended in two banks of four and broke with pace in Tuesday's EFL Cup semifinal, are a case in point. They didn't just aim to frustrate.
Many of the worst efforts -- Watford, when losing 6-0, Stoke, when going down 7-2, and Manchester United when flattered by a 2-1 scoreline after fielding four attack-minded players but with two of them appearing auxiliary full-backs as they struggled to get attacking possession -- involved using the division's default formation in recent years, a 4-2-3-1.
Klopp might note that the one side to hold an 11-man City in the league, Crystal Palace, did so using the 4-3-3. Roy Hodgson's side secured a stalemate and even came closest to winning after he swapped Wilfried Zaha to the right flank to get him running at Danilo. United threatened to have similar joy if they could have shuttled the ball over to Anthony Martial so he could attack Fabian Delph. Yet if there is a theory that City's left-backs are a weak link, especially in the absence of the injured Benjamin Mendy, few have managed to expose them.
Palace also prospered by using Christian Benteke as a target man and bypassing City's pressing with direct balls. Hodgson was bolder than many of his counterparts in his own pressing game. Bristol City (who scored as an indirect result of closing down Eliaquim Mangala), Palace, Liverpool, Burnley (who performed better than 3-0 and 4-1 scorelines indicate) and Tottenham tried to close down City's defenders and stop attacks at their source. Others have retreated immediately and sat deep, though none as far as Newcastle with their blanket defence.
Other strategies have been trialled. Huddersfield got striker Laurent Depoitre to try to mark Fernandinho, aware of the importance of the holding midfielder in Pep Guardiola's passing game. That approach was soon copied; so too was Southampton's ploy, after Mauricio Pellegrino's side came within seconds of taking a point after played a 3-5-1-1. David Moyes, though usually a devotee of a back four, copied it and, after West Ham performed respectably in a 2-1 defeat, kept it to beat Chelsea.
Pellegrino benched Steven Davis, Dusan Tadic and Sofiane Boufal, highlighting a concerted approach by Southampton. Technical attacking midfielders, particularly slower ones, tend to be omitted by managers who expect to operate without the ball. Instead, they have constructed two blocks of three: a trio of centre-backs protected by another of defensive midfielders -- a riposte to City's excellence in midfield, dominance of possession and capacity to get players into central areas. Tony Pulis, who was willing to field three defensive midfielders anyway, needed no encouragement even if West Bromwich Albion's 3-2 defeat was illogically entertaining.
When Mauricio Pochettino looked to get four men in the heart of the midfield by playing a diamond, it backfired by leaving Tottenham's right-back Kieran Trippier exposed against Leroy Sane in a system that lacked width in front of him. It's part of the problem against City: Guardiola stretches games with wingers. When Chelsea played a 3-5-1-1, Sane and Raheem Sterling turned it into a back five, creating more room in midfield for the eventual match winner, Kevin de Bruyne.
Of course, defending champions Chelsea were beaten 1-0 at Stamford Bridge, and only one side has started with a back three and taken a point against City this season: Everton. The confusing element, apart from the fact that the game's tactics changed following Kyle Walker's first-half dismissal, was that Ronald Koeman changed to a back four after an hour and with his side 1-0 up. That was also when City were starting with a 3-5-2 formation and reacting to a red card.
These days, Guardiola's favourite 4-3-3 is firmly entrenched. It has been altered only when he has countered opponents' dullness by moving Fernandinho into defence or switching to a 4-2-4 or 3-3-4, though that was only possible when both Gabriel Jesus and Sergio Aguero were fit. If such defensive tactics have frustrated Guardiola, Klopp is unlikely to copy them. He has five victories against Pep, generally prospering with pressing and attacking.
If City have found a way to win against the most negative sides, now it is a question of whether the most positive can achieve the feat that has so far eluded everyone else and beat them once and for all.
Richard Jolly covers the Premier League and Champions League for ESPN FC. Twitter: @RichJolly.