Chelsea 3-2 Arsenal: 4 Key Takeaways from this season's first game at Stamford Bridge

Wow. What a game it was!

Chelsea defeated Arsenal 3-2 with Marcos Alonso's 81st-minute winner proving to be the difference in the London derby.

The clash at Stamford Bridge, Maurizio Sarri's first at home, began at a frantic pace and it only took nine minutes for the hosts to get in front through Pedro. Alvaro Morata got off the mark for the season 11 minutes later after getting on the end of Cesar Azpilicueta's long ball once again.

Henrikh Mkhitaryan halved the deficit on 37 minutes and created Alex Iwobi's equaliser four minutes later.

Eden Hazard's influential impact off the bench was then the game changer for the Blues, the Belgian beating Lacazette down the line and putting it into the box for Alonso to guide into the net.

Here are four takeaways from the game.

#1 Helpless in defence

With both sides clashing under new managers at the helm, it's fair to say that both bosses need to work on the defensive shape of their team.

Arsenal's concerns with protecting their goal has been common knowledge of its spectators, whereas Sarriball is prone to leaving gaps at the back.

The Gunners' fullbacks were consistently done by balls over the top for Chelsea's runners, with Bellerin a particular liability when he makes a push up the field.

Chelsea, who are associated with conservative football, also had problems with the positioning of their fullbacks. The adventurous Alonso was caught out by Bellerin on a few occasions and even the ever-reliable Azpilicueta failed to track Iwobi into the area for Arsenal's second.

#2 Guendouzi is the real deal

One Arsenal player who can take some sort of pride away from their Premier League start is Matteo Guendouzi.

The Frenchman made a promising display against Manchester City last week and continued to impress when matched against Jorginho and Kante in Chelsea's midfield.

Playing alongside Granit Xhaka, Guendouzi's performance was levels above his senior, with the Swiss international hooked at half-time.

Guendouzi, on the other hand, made the most tackles and interceptions for the Gunners, whilst also supporting those in front of him with passes breaking the lines. In the end, Guendouzi had a pass accuracy of 93%.

#3 Difficult start cannot define Emery

Unai Emery must be ruing his luck having been drafted to face Manchester City and Chelsea in his two introduction games to the Premier League.

Arsenal do not come up against another of their top six opponents until November, when they welcome Liverpool to the Emirates, allowing Emery at least two months to build momentum and impose his philosophy onto his new side.

Expect a more assured Arsenal side in the next few games.

#4 Hazard is irreplaceable for Chelsea

No matter how much money Real or another side throw at Chelsea for the services of Hazard, they're unlikely to obtain anyone on the same level to replace the Belgian.

Hazard's introduction to the game leaned Chelsea closer to the win, with him constantly picking up the ball on the turn and charging into the final third.

As he did last week when he made a substitute appearance, Hazard left the field having collected an assist for Chelsea's third goal of the game.

Tying him down long-term must be the priority for the Blues as he's an essential cog in Sarri's machine.