Manchester City's 2-0 victory over Manchester United may prove to be the decisive moment in the title race, as Pep Guardiola's side overcame their final serious test.
They did it, however, without John Stones. For a game against a counter-attacking United side boasting little else other than speed upfront, Guardiola chose to field the somewhat immobile Vincent Kompany alongside Aymeric Laporte. On the bench, meanwhile, was Nicolas Otamendi. Stones didn't even make the 18-man squad, with no suggestion he's injured.
It's more likely that Stones was dropped after a dreadful showing against Tottenham at the weekend, where goalkeeper Ederson was forced into his most prominent display of the season, constantly thwarting Son Heung-Min. Stones repeatedly found himself bypassed, getting sucked towards play and then being beaten by midfield runners.
Stones' disappointing campaign is in line with that of the other players used in defensive positions during England's impressive run to the World Cup semifinals last summer. What appeared a fresh new generation of talent has already started to look creaky.
Perhaps the most obvious example is in goal, where Jordan Pickford excelled in Russia last summer. His performances at Everton this season, though, have been hugely concerning.
Pickford has been poor when sweeping outside his goal this season. For example, back in August he was fortunate not to be dismissed for a high boot on Southampton's Danny Ings, while in a 6-2 thrashing at home to Spurs just before Christmas, he found himself hopelessly out of goal when rounded by Son for Spurs' opener.
He's also struggled with aerial balls, including in a 2-0 defeat to Manchester City. Not exactly renowned for their tendency to cross, they seemingly spotted a weakness in Everton's goal, while his double-error away at Newcastle, spilling a cross and then immediately hauling down Salomon Rondon, was another moment where he was fortunate not to be shown a red card.
His most famous mistake, for Divock Origi's late winner at Anfield, was perhaps unfortunate, and there have been matches, against Manchester United, Newcastle and Crystal Palace to name a few, where his goalkeeping has been impressive. That said, there's a sense that this is a return to Pickford's true level -- sporadically brilliant but frustratingly erratic -- and that his World Cup showings were something of a flash in the pan.
Something similar could be said of Kieran Trippier, who has endured an even worse campaign for Tottenham -- on the back of an excellent World Cup, where his free kick against Croatia put England on the brink of the World Cup final. His first goal of this campaign, a David Beckham-esque free kick in an early season win over Fulham, was a reminder of his expertise.
Defensively, though, he's struggled. He made an error for Roberto Firmino's goal in a 2-1 loss at home to Liverpool in September, and struggled badly against Solly March the next weekend at Brighton. He also made a major error to let in Raheem Sterling for Riyad Mahrez's winner for Manchester City at Wembley, and his questionable positional play left Juan Foyth badly exposed in a 3-2 win at Wolves, where the young Argentine conceded two penalties.
Add in another poor performance in the reverse meeting with Wolves, the concession of possession for Marcus Rashford's winner in Manchester United's 1-0 victory over Spurs at Wembley, and a crazy own goal against Chelsea, and it's been a disastrous campaign. Tottenham don't make many signings, but a new right-back is a priority -- Trippier is an acceptable right wing-back, but struggles badly in a four-man defence.
England effectively deployed two right-backs in Russia, with Kyle Walker tucked inside into a back four. Again, however, it's arguable that Walker has endured his worst campaign for several years. He struggled on his return to Tottenham back in November, and was particularly culpable for City's poor display in the shock 3-2 home defeat to Crystal Palace shortly before Christmas.
At times Walker has been asked to play an unusual role, drifting inside into central positions rather than overlapping, and he's sometimes found himself exposed when City's possession play isn't up to scratch. But his display in the first half at Old Trafford last night, full of misplaced, overhit passes, summarises his current malaise.
A more impressive performer was on the opposite flank. Ashley Young's transformation from a creative wide player into a versatile, jack-of-all-trades defensive operator has been somewhat unexpected, and it's difficult to criticise a player who has manfully coped wherever he's been fielded. Now almost a right-back by trade, Young played left wing-back for England at last summer's World Cup, though has not been called up since, and has recently been used as a right-sided centre-back by Ole Gunnar Solskjaer.
Young is actually a competent right-back -- he was Man of the Match when up against Eden Hazard in a 2-2 draw at Stamford Bridge earlier in the season -- but in recent weeks he's looked fatigued, and a couple of moments of indecision against Liverpool and Barcelona have underlined the fact United need an upgrade in his position. At 33, his international career is probably over.
Playing just inside him last summer was Harry Maguire, another who started the campaign positively with a superb long-range, last-minute winner against Southampton. But he's also another who has struggled in the second half of the campaign, dismissed in the first five minutes against Burnley for a last-man foul on Johann Gudmundsson, outjumped by Ryan Bennett in a 4-3 defeat to Wolves, and often getting into a muddle with Wes Morgan, with the two struggling to cope with balls played into the channels.
And then there's Stones, who started the season impressively, and was often tasked with drifting into different positions as City's shape evolved: he played periods at right-back, and periods in central midfield. But Stones still hasn't yet developed into an undroppable regular, and for all his positivity in possession there are lapses of concentration, defensive errors and moments where he finds himself in a poor position. Stones might actually be responsible for the season's most defining moment -- when he hacked the ball off his own goalline in the crucial 2-1 win over Liverpool in January, although that situation had only arisen when he'd thumped a clearance straight at Ederson, his own goalkeeper, and the ball looped towards goal.
The positive news for England, though, is that there have also been emerging performers. Trent Alexander-Arnold has been the best right-back in the league and is now ahead of Trippier and probably Walker in the pecking order, while Crystal Palace's Aaron Wan-Bissaka has been unlucky not to receive a full call-up. Joe Gomez, aside from injury, has looked commanding in central defence, and the partnership between those two could work nicely for Liverpool and England over the coming years. At left-back, Luke Shaw still makes mistakes but has enjoyed his most consistent period of starts for Manchester United, while Leicester's Ben Chilwell has been excellent, and made his international debut at his home stadium last November.
Also significant is the fact Southgate has moved away from the five-man defence, and reverted to a back four. While partly because of tactical concerns, and England's tendency to find themselves outnumbered down the flanks, perhaps there's also a secondary explanation -- with so many of last summer's defenders struggling for form, it's something of a relief to only need to find four defenders, rather than five.