Alex Cable analyzes how Aston Villa dealt a huge blow to Arsenal's title aspirations with a 2-0 win at the Emirates Stadium.
If Arsenal don't win the Premier League this season, Villa, the only team to beat them twice, will be responsible. Unai Emery, of all people, is the one who has done the most damage.
Villa well deserved the three points after a brilliant performance, from going deep in the first half to turning the game on its head and dominating the second half.
It is a tactical victory for Emery, but it was equally a tactical defeat for Mikel Arteta, whose team selection and formation have proven to be very attacking.
The man at the center of it all, for better and worse, was Kai Havertz: the key to Arsenal's good first half and the symbol of Arsenal's second-half collapse.
Havertz's intelligent positioning is the key to early happiness
Arsenal were on top in the first half, pinning Villa in a low block which made it difficult for the visitors to counter-attack and ensured the home side dominated.
Chances were few and far between, and indeed Villa came as close to taking the lead as Arsenal, but there is no doubt that Villa were on the back foot – thanks mostly to the sudden use of Havertz in the No.8 position.
Villa, in their flat 4-4-2 shape, did not have the numbers to cope with Havertz's runs into the left halfspace, and with Leandro Trossard pulling Ezri Konsa wide, the German constantly found the gap driving into.
In this example, the ball was passed from Havertz to the goal.
It happened time and time again, with John McGinn unsure how to close the space, and by the end of the first half Arteta must have thought the goal, via Havertz (circled in black), was coming.
The downside of playing with Havertz as a number 8 appears in the second half
After the break, Villa dominated that area, with McGinn (circled in black) dropping deeper to become part of the back five, closing down Havertz's space.
But that's not why Villa won. They did this because once they started passing the ball out from the back and getting a foothold, the problem of playing with Havertz in midfield began to appear.
Havertz and Declan Rice formed a two-man midfield when Villa had possession, which was frankly too light for space to defend, especially with Morgan Rogers effective at dropping the ball to receive the ball between the lines.
Havertz completed one tackle and no interceptions, the same as Jorginho did in just 20 minutes and significantly fewer than Youri Tielemans (four tackles, one interception) and McGinn (three tackles, three interceptions).
Worse still, once Villa sat higher up the pitch, Arsenal lacked someone to control the tempo from deep areas or play progressive passes to the strikers.
In other words, they missed Jorginho.
By contrast, Tielemans and McGinn were brilliant Villa players, evading pressure and putting things together with the finesse and intelligence that Arsenal lacked.
It was all about that midfield, which was tilted in Villa's favor when the territorial battle became evenly matched; Villa had 43.7 per cent of possession in the first half, but between the 46th and 87th minutes, when he scored his second goal, they had 61 per cent of possession.
Once that happened, Arsenal's main strength in the first half – an unusual attacking line-up led by Havertz – became their weakness.
Emery's subs win the day as Arteta reacts too late
Nervous, deadlocked at 0-0, and with the dominance of one team replaced by the other, substitutions were always going to play a big role.
But you thought Arteta would be the one to turn things around.
Instead, he waited too long to bring on Jorginho, preventing Arsenal from regaining momentum, and when he initially made the substitutions in the 67th minute, he sent off the wrong full-back.
Oleksandr Zinchenko looked shaky throughout the half. In fact, Emery brought on Leon Bailey in the 61st minute to put him under pressure.
It worked immediately – and eventually led to the opening match.
Bailly (who has now contributed seven goals from the bench, more than anyone else in the Premier League) scored the first goal of the second phase from a corner, but the move that led to it began with a clever pass from Villa's right with Zinchenko under pressure.
In hindsight, Arteta made a mistake in his changes as well as his initial selection. It was always going to be a risk to field so many strikers – and keep out the in-form Jorginho – when facing such a high-quality opponent.
On the other hand, Emery did everything perfectly. It was a tactical skill that removed any creeping doubts about Villa's recent form – and raised serious doubts about Arsenal's title hopes.