Manchester City's dream of a quadruple is over after Monday night's shock 1-0 defeat to Wigan Athletic. While this was the League One side's third straight win in FA Cup games between the two clubs, few would have predicted this Wigan side would overcome Pep Guardiola's hitherto all-conquering superstars.
Many, though, might have guessed the identity of City's fall guy -- Fabian Delph -- who was sent off for precisely the type of dangerous lunge about which Guardiola has complained in recent weeks. Even with 10 men, the Premier League leaders should have still triumphed against such inferior opponents, but their left-back's error was crucial in making the tie more balanced.
Delph is not, by any stretch of the imagination, a natural in the position. However, in the absence of Benjamin Mendy and with Danilo sometimes required in his more natural position on the opposite flank, the England international has performed reasonably well in that position for someone more used to operating in central midfield.
It helps that Delph is playing in such a well-organised side that dominates possession but, despite that, he is nevertheless City's weak link when played at left-back; opposition managers will study Guardiola's side and decide the right wing is their best chance of getting some joy.
But City's left-back weakness is part of a pattern that extends to other top sides in England. This isn't a golden age for defenders, but there remain several top-class centre-backs, while the likes of Kyle Walker, Hector Bellerin and Antonio Valencia have all established themselves as dependable on the right.
Across Manchester, Jose Mourinho hasn't settled on a regular left-back during 18 months in charge at United. Instinctively it feels like the defensive-minded Matteo Darmian is the best fit, but his positioning -- getting sucked inside too much and leaving space on the overlap -- is often dreadful. Daley Blind seems the antithesis of a Mourinho player, despite sometimes excelling in that position under other managers, so this season it's been either Ashley Young or Luke Shaw.
Like Delph and another former Aston Villa player, James Milner, Young is a reluctant convert to full-back and yet, against top-class wingers, he sometimes struggles in one-against-one situations. Shaw, meanwhile, never seems entirely certain of his status at Old Trafford; he has been criticised by Mourinho after playing well and praised by his manager, only to be dropped.
Shaw remains a good overlapper and is comfortable in defensive zones but, realistically it's difficult to say he's a better player than four years ago, when he was taken to the World Cup ahead of Ashley Cole -- probably England's best-ever left-back -- and earned his move to United. A horrific leg break was a major setback, but it's difficult to predict where Shaw will be in two years: a regular at United, or playing for a mid-table club.
Liverpool's situation looks more certain after some excellent performances by Andrew Robertson, particularly his shackling of Raheem Sterling in a 4-3 win over City last month. But Robertson's "regular" status is relatively recent and it's worth remembering that Alberto Moreno started the season well until a disastrous performance at former club Sevilla in a 3-3 draw. Robertson mixes a good left-footed delivery with tackling, but to consider him a truly top-class performer at this stage would be premature.
There's more of a question mark at Tottenham, where Danny Rose has barely been a regular over the past year. Ben Davies has played more frequently this season, but the Welsh international doesn't offer the dynamism that makes Rose such an attacking force. Davies is solid defensively but lacks speed, a limitation in an attacking and a defensive sense, as well as a weakness that means he doesn't quite look right for a Mauricio Pochettino side.
Similarly, Monreal is enjoying a good campaign with Arsenal and earning overdue credit in part because he's suddenly discovered an ability to score goals. But he is probably more suited to playing on the left of a three-man defence, where he was generally Arsenal's most positionally faultless player, rather than in a conventional left-back role.
Arsenal have always depended upon more mobile, quick players in wide defensive areas, such as Cole, Gael Clichy and Kieran Gibbs. Meanwhile, the situation regarding Sead Kolasinac is also peculiar: After being a regular in the opening weeks of the season, he suddenly fell out of favour and, while a formation switch didn't help, Arsene Wenger even preferred to use young central midfielder Ainsley Maitland-Niles.
Finally there's Chelsea, who have spent most of the last 18 months using wing-backs rather than full-backs, which perfectly suits Marcos Alonso, a player signed precisely because he was accustomed to playing that role at Fiorentina. His play is often littered with errors but he remains excellent in somewhat unusual situations for a player in that position and also scores goals, particularly with his head.
His dead-ball expertise is also of great value but, nevertheless, Antonio Conte -- and / or Chelsea -- don't seem entirely satisfied; they repeatedly sniffed around Juventus' Alex Sandro, before instead signing Emerson Palmeri in a move that suggests Alonso's place is far from secure.
Outside the top six, it's tough to find many star performers. Everton's Leighton Baines was once the best in the league, but has looked increasingly immobile and suffered from injuries. It means right-back Cuco Martina has been forced to play out of position, often with disastrous results.
Elswehere, West Ham's Arthur Masuaku sometimes shows promise going forward but is defensively suspect, while Leicester's Christian Fuchs is solid, but not quite as good as his performances in a title-winning season two years ago would suggest. Perhaps the only real star is Ryan Bertrand, a truly excellent full-back defensively and offensively and far too good to be involved in a relegation battle at Southampton.
There are two obvious conclusions. The first is that one of this summer's main transfer targets will be Fulham's Ryan Sessegnon, among the star players in the Championship at left-back despite being just 17. Likely to be wanted by almost every big Premier League club, his transfer value could be quite staggering.
The second is that, though the Wigan game showed their limitations at left-back, City will pull further away from the chasing pack when Mendy returns with his tremendous stamina and precise low crosses. By the end of the campaign, a position of weakness could be one of their major strengths.