Arsene Wenger will never forget his 1,000th game in charge of Arsenal - but, unfortunately, for all the wrong reasons.
The Frenchman watched on helplessly as old adversary, Jose Mourinho, masterminded a 6-0 demolition of the Gunners at Stamford Bridge.
Goals in the opening seven minutes from Samuel Eto’o and Andre Schurrle left the visitors with a mountain to climb and things went from awful to downright dreadful - and not to mention farcical - eight minutes later when Kieran Gibbs was incorrectly sent off following Alex Oxlade-Chamberlain’s handball on the goal-line.
Eden Hazard converted the subsequent penalty before Oscar scored a goal either side of half-time and Mohamed Salah completed the rout with 20 minutes remaining.
Fast-forward two-and-a-half years, though, and things are now very different.
Wenger secured a 3-0 victory over the Blues at the Emirates Stadium on Saturday evening - and there’s now genuine belief that this Arsenal side will challenge Pep Guardiola’s Manchester City, and Mourinho’s United, for the Premier League title.
But what has changed over the past 30 months? Let’s take a closer look…
Jose Mourinho is no longer at Chelsea
Wenger’s record against Mourinho isn’t great; in fact, it’s pretty appalling.
In 13 Premier League meetings between the two managers, Wenger has never won. Not even once.
Mourinho has secured victories on seven occasions, while the other six meetings have ended all square.
There’s no two ways about it: with Mourinho off the scene, Wenger’s chances of beating Chelsea increase significantly.
Arsenal now have a decent goalkeeper
And not just any decent goalkeeper: the goalkeeper who was in net for Chelsea when Arsenal suffered that demoralising defeat at the Bridge in 2014.
Petr Cech is a huge upgrade on Wojciech Szczesny, who now plies his trade with AS Roma.
The 34-year-old instills confidence into the Arsenal back-four and this spreads to the rest of the team.
Szczesny, on the other hand, tended to make Arsenal’s defenders nervy and on edge because of his unpredictable nature and penchant for the ridiculous.
Arsenal’s attacking line-up is so much stronger now
Against Chelsea in 2014, Arsenal had Olivier Giroud flanked by Lukas Podolski and Tomas Rosicky, with Oxlade-Chamberlain and Santi Cazorla pushing up from midfield.
Against Chelsea at the weekend, Arsenal had Alexis Sanchez leading the line, with Mesut Ozil behind him and Theo Walcott and Alex Iwobi completing a pretty formidable front four.
Sanchez and Ozil, when on song, are capable of dismantling the Premier League’s best defences.
Chelsea’s defence was decent back then
In the 6-0, Chelsea lined up with John Terry and Gary Cahill in central defence, with Branislav Ivanovic and Cesar Azpilicueta in the full-back positions.
All four players are still with the west London outfit, but they’ve all regressed since 2014.
It was the same back four against Arsenal - with the exception of the injured Terry, who was replaced for this fixture by David Luiz - and they were torn to shreds by Sanchez, Ozil and co. at the weekend.
Arsenal are learning to keep their discipline
There have been numerous examples - including the 6-0 against Chelsea - of Arsenal conceding early goals and eventually losing players out of sheer frustration over recent years.
It was a slightly different story against the Blues in 2014 - Gibbs was sent off by referee Andre Marriner following Oxlade-Chamberlain’s efforts to prevent a third Chelsea goal from going in - but they still shot themselves in the foot.
Against Chelsea on Saturday, Arsenal didn’t pick up a single yellow card.
Arsenal learning how to beat the big teams again
Last season, Arsenal recorded Premier League victories over Tottenham, Manchester United and Manchester City. They were also the only team to do the double over the eventual champions, Leicester City.
Wenger’s men got it horrible wrong against Liverpool on the opening weekend of the season, but Saturday’s win lends more weight to the argument that Arsenal are finally learning how to perform against - and beat - the big teams again.
Arsenal’s bench is now so much stronger
Arsenal’s bench in 2014: Vermaelen, Flamini, Jenkinson, Fabianski, Sanogo, Kallstrom and Gnabry.
Arsenal’s bench in 2016: Xhaka, Gibbs, Giroud, Lucas Perez, Ospina, Oxlade-Chamberlain, Holding.
It’s patently clear that Arsenal now possess more strength in depth.
After Francis Coquelin got injured in the first half against Chelsea, on came the Gunners’ £30 million summer signing Xhaka.
It just goes to show that two-and-a-half years is a lifetime in football terms.
Can Arsenal win the Premier League title? Have your say by leaving a comment below.
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