The Gunners won a breathtaking 2-0 English Cup victory at Wembley, as their coach repeatedly drilled holes in the back line of his former mentor.
Manchester City has suffered their first domestic cup defeat in 880 days. One could generally attribute this to circumstances; all good things will surely come to an end.
With two League Cup wins, one FA Cup win and a total of eight wins at Wembley, City were likely not expected to lose.
Although they had 71% possession of the ball in their FA Cup semi-final loss to Arsenal and beat the Gunners 16-4 in shots on target, they were unable to break through a dynamic and committed defense.
Mikel Arteta has clearly learned a lot from his two seasons alongside Pep Guardiola at the city dugout and has thus outsmarted his former teacher.
The Arsenal boss drilled his side to reduce the sideline cuts that led so many city goals to close the room to prevent Kevin De Bruyne and Ilkay Gundogan from moving the ball quickly, and closed the spaces near the penalty area where David Silva weaves his magic.
It was not a loss due to bad luck or missed opportunities. Guardiola chose his strongest line-up minus injured Sergio Aguero and finished second.
City's problems have been exposed and the defeat has highlighted areas where Guardiola wants to strengthen in the transfer market when the window opens in a week's time.
There was a worrying trend in the recent road losses to Manchester United, Chelsea and Southampton. These games give City a lot of possession, but they still haven't managed to convert the odds of the open game. Of these 16 shots against Arsenal, only one tested goalkeeper Emiliano Martinez.
Most concerning, however, is their inability to close the back door in games where they dominate possession.
Guardiola's high-risk, high-reward strategy of playing high on the pitch relies on an alarm defense to take out all threats before they start.
Arsenal's victory exposed the weaknesses of a back line that isn't fast enough or smart enough to deal with sharp counterattacking football. It's an issue throughout the season that was revealed by quick sides like Wolves and United at Etihad Stadium and relegated Norwich back in September.
Arteta's Arsenal were precise and courageous when they played from behind, but City were slow when the high press was broken.
Benjamin Mendy gave Nicolas Pepe too much room to swing on a cross for the opening goal, and Kyle Walker was too loose to keep it from reaching Pierre-Emerick Aubameyang for his excellent finish.
Aubameyang had set his race perfectly and then sprinted out of defense to roll in the second. He might have had a hat trick if he had taken advantage of an opportunity Ederson had given him after a poor pass.
Pep Guardiola admitted: “We did not play good and on a day when you had to, we did not. "We started low and without the way you have to play. We are always ready many times but today we were not.”
However, the City manager has few options to turn to.
Walker has been largely consistent throughout the season, and Joao Cancelo, who was signed this summer, couldn’t come close to replacing him as a first-choice right-back. On the other hand, Oleksandr Zinchenko's form deteriorated after becoming a regular in the incredible title defense last season.
Center defenders Nicolas Otamendi and John Stones have little self-confidence and struggled to impress with a narrow midweek win over relegation-prone Bournemouth. Guardiola currently plays 19-year-old rookie Eric Garcia in that position alongside Aymeric Laporte.
Guardiola is sure to strengthen this summer with the possibility of two new center-backs and a full-back on his wanted list.
Before that, he must find a way to relieve the pressure of a well-drained back four if they are to sneak past Real Madrid in the Champions League round of 16 and the mini-tournament in Portugal at the end of the season.
Guardiola added: “We’re struggling to play in our level. “You don’t have to be a genius to realize that we need to raise our level against Madrid.
Zinedine Zidane, who already has another La Liga title in his pocket, will have looked with new hope and excitement before his trip to the Etihad, even if his team has to overcome a deficit of 2-1..