AC Milan and Arsenal were in both similar and yet oh-so-different positions as the Europa League Round of 16 kicked off.

Both are European giants currently on downswings, but Milan hadn’t lost in 75 days. Arsenal hadn’t lost in four.

Arsenal hadn’t scored multiple goals in its last four matches. Milan hadn’t conceded any in its last six.

So naturally, then, by the throw-up-your-hands logic of soccer, Arsenal strode into the San Siro on Thursday and put together arguably its best performance of the season in beating Milan 2-0 to take a decisive lead in the first leg of their tie.

Milan has been a much better outfit since club legend Gennaro Gattuso took over for the sacked Vincenzo Montanella on Nov. 27. Despite a rocky start, Milan has found its form behind the hard-charging but tactically sound manager.

[ Follow FC Yahoo on Twitter and Facebook ]

Only those tactics may have inadvertently led to the team’s demise against Arsenal. Part of Gattuso’s plan was to give his fullbacks and midfielders more freedom to make sudden runs forward, meaning they’ve had to learn to cover for each other in sometimes atypical areas. It’s been a sound strategy in the more defensively rigid Serie A, but it still requires familiarity and communication, and Arsenal punished the work in progress.

For all the guff Arsenal has (rightfully) received in recent weeks for its slump, the side it started on Thursday is rife with European experience, most notably Mesut Ozil, Henrikh Mkhitaryan and Danny Welbeck. That trio wrought havoc on Milan’s back four most of the first half with darting movement and quick counters, and Ozil picked out Mkhitaryan 15 minutes in for a fortunate but deserved opening goal:

That back four, which includes fullbacks Davide Calabria and Alessio Romagnoli and veteran steward Leonardo Bonucci, is expected to form the shell of the Italian national team’s defense for years to come. But against Arsenal, Calabria and Romagnoli showed every bit of their 21 and 23 years of age, respectively, while Bonucci looked more like the lost center back he was earlier this season.

The second goal came shortly before halftime when Ozil played through Aaron Ramsey with another sumptuous ball, which Ramsey took across goalkeeper Gianluigi Donnarumma (another Italian starter-in-waiting) cool as you like to double the lead:

The second half saw Milan generate more chances, but rarely with the teeth of Arsenal’s. The Italian side doubled the visitors’ shots 16-8, yet the Gunners had three more total on target. Moreover, Arsenal’s defense looked uncharacteristically sturdy and sure-footed for 90 minutes, leaving Milan vexed and out of answers.

Perhaps the biggest difference of the night was forced upon Arsenal. Big-money striker Pierre-Emerick Aubameyang is almost completely reliant on service into him and doesn’t link up as well with teammates as, say, Welbeck, and has therefore struggled to excel since arriving at the end of January. He’s not allowed to play in the Europa League because his previous club, Borussia Dortmund, is also in the competition, which turned out to be a break for the Gunners.

It’s an effort few expected out of Wenger’s side, and the calls for the club to change managers like Milan did have grown almost deafening. There has, however, always been a resilience to Wenger, an ability to snatch a big result when it looked like another bad one would sink his time at the Emirates for good.

That’s what happened Thursday. And now, with both teams well off the top-four pace domestically, it’s Arsenal that has a leg up in the competition that offers the best chance to return to the Champions League.

It’s Arsenal, with its old manager, heading into the weekend with confidence. And AC Milan, with its new manager, is left to rediscover some.

Joey Gulino is the editor of FC Yahoo and moonlights as a writer. Follow him on Twitter at @JGulinoYahoo.