The Arsene Wenger era at Arsenal was all about the style of play, often at the expense of tactical flexibility. Unai Emery has been the exact opposite.
Downfall

This ultimately led to his downfall when, in his final years, Arsenal did not have the caliber of players to impose their will on the other side.
Unai Emery throughout this season has been the exact opposite.
When the whole squad is healthy, it is clear that he is capable of, and even desires to play “Wengerball.”
The team goal against Fulham that Ramsey finished is one of the more beautiful sequences of the last 10 years from the Gunners. Fans sang out, “We’ve got our Arsenal back” in one of the more joyous moments of the season so far.
https://www.youtube.com/watch?v=79sZhFf6On8
But in more recent games, fans have been frustrated by a more “ugly” and defensive style of play.
But why is that?
Did Unai see the Fulham goal and get disgusted?
Absolutely not.
The more defensive shape has been necessitated by squad fitness and form.
Let me explain…
Poor fitness and poor form

Arsenal had Aubameyang, Sokratis, Bellerin, Holding, Özil, Lichtsteiner, Xhaka, and Welbeck all out for various reasons at Huddersfield. That’s almost a full XI and a lot of attacking talent left in London.
Both right backs in the squad being out also put Arsenal in a bind with defensive options.
Sokratis being out meant that Mustafi—maybe the most criticized player in recent Arsenal memory—had to play unless Emery wanted to field only one true central defender (Koscielny) or a young player who has not played first team minutes all season (Mavrapanos).
Arsenal had no true right back, a central defender that is a comedy of errors, and a left back that is much better going forward than in defending (Sead Kolasinac). Not exactly a back line that strikes fear into the hearts of Premier League opponents, however low on the table they might be.
So what was Emery to do?
Plough ahead, pretending a team this far short of first choice still had the quality to impose their will in the style of Arsene and end up dropping points at the worst team in the league?
Lemons out of lemonade

Instead, Emery chose to take a long look in the mirror.
The squad available to him on Saturday was never going to give anyone a 5-0 thrashing via an attacking masterclass. So he did what would make Arsene weep: he fielded a back 5 with 2 defensive midfielders in Torreira and Guendouzi against the bottom-placed team in the table.
It was by no means pretty, but it got the job done.
The more defensive shape was able to cover for Mustafi, putting him in a better position to succeed with two defensive midfielders in front of him. Mustafi put in six clearances and an astounding five interceptions, more than any other player.
Having three central defenders also allowed Kolasinac to stay forward more and prevented Ainsley Maitland-Niles (who is not really a defender at all) from being isolated against Jason Puncheon on the right.
That help undoubtedly made Maitland-Niles more confident to go and win a tackle, as he did six times. That was the highest of any player in what was a Man of the Match performance.
He also chipped in with two clearances, two interceptions, and an assist going forward.
Substance over style

It was not a defensive performance that will make fans cry out for this to be the new back line. But it put the players that were available into a position in which they could succeed, handing Arsenal a critical three points in a top four race that they are not out of yet.
That is the hallmark of a good manager.
These effective managerial decisions made many fans feel like Emery ONLY adapts to the opponent and doesn’t have his own style. But is that really true? His hand at Huddersfield was forced into that lineup and tactical plan due to injury and form.
Emery’s ability to get results in the face of squad depletion due to injury, being forced to use out of form and out of position players shows a tactical flexibility that is more than welcome at Arsenal.
With a little bit of board backing in the transfer window, this manager could return Arsenal to the heights where they belong.
Statistics courtesy of WhoScored.com