The Benfica game sure did feel retro, didn't it? I mean, we're back to a single striker after a full season of a (superficially, of course) very old-school striker duo, we employed the underlapping inverted fullback system of the Man Utd 2014 vintage, and Xabi Alonso played at a level you wouldn't think possible from a, *ahem*, grandpa of his age. But let's take it one at a time.
Firstly, Alonso was immense in his playmaking role. Absolutely not content just passing it around at the back, he completed a whopping 16 long balls in the game (out of 19!). This had a lot to do with Pep's preferred gameplan which I'll explain as the post goes along - keep in mind it's all intertwined. Using Costa in that inside forward role has had its strengths and weaknesses - the strength being that one amazing cross he gave to pull us back v Juve, the weakness being those other 14 crosses that didn't connect with anyone - so building the play around setting him up in positions to come inside at a distance where shooting is an impossibilty felt like an odd decision to me. Many times he was forced so far back when trying to cut inside - I remember him having to retreat with the ball all the way to the center circle at one point. But I can't help but feel that his forays into the center of the pitch were all a feint that served a bigger purpose in the general game plan.

So much to talk about, I'm honestly not trying to be this inconcise! Anyway, here we have a classic Pep situation - one flank is overloaded, creating space out wide on the other flank. A quick ball back to Xabi Alonso and a glorious pin-point pass to Douglas Costa out wide on the right. Suddenly the situation is entirely different, as Costa is in (relative) space with the ball, with his opposing fullback and wide midfielder racing to double mark him as is tradition in this second half of the season. This is the decision point in the play, and boy does he have a lot of options. The first one that comes to mind - speed towards the touchline and toss it in the mixer! It's direct, and Costa is quick and everything, and can, when needed, deliver a cross with the outside of his boot. But the question - why? What's the point of a touch-line cross against Lindelöf and Jardel, 1.87 and 1.92 tall respectively? No, in this game we saw Costa mostly take two options.

Option one: Costa cuts in, draws players to him. Lahm is positioned conservatively just behind him, for an emergency backpass or to sweep up the remains of a failed dribble, so defensively this is a very good situation. Costa keeps the ball long enough to have the Benfica team drift towards the center-right of the pitch to prevent possible combination plays by Müller and Vidal, and then BOOM long ball to Ribery, completely isolated down the left flank. Ribery then, being in a far better position than he could hope for if the team used brute force down the flank, has pretty much free reign to rush towards the penalty box-touchline area where he so excels in quick crosses and cutback. Alternatively he can pass it to the over/underlapping Alaba. In either case, the team is far better positioned to exploit the low cross from quite near the goal than it would an overlapping cross from near the corner flag.

Option two: as Costa is double-marked, Lahm drifts into the half-space which is now relatively unguarded, as the central midfielders are also occupied with Vidal and Thiago, the forwards seek to maintain positional discipline to enable counterattacks and the centerbacks have their hands full with Müller's incredibly annoying movement. And suddenly Lahm is in possession in the penalty area with the Benfica team scrambling back to cover.

The specific positions of the players are irrelevant. Müller could be pulling away from the central defenders (which he did during one attack), Vidal coul be just outside the box (where he scored the goal from following a Lahm underlap), Alaba could be charging into the box from afar (which he also did). The important thing is that Lahm, a player with immense vision and intelligence, is in possession almost near the six-yard box, with his head up and his team creating movement in the penalty box. In poker terms, this is close to a straight flush in just how good the situation is.
So why go through this complicated (well, not really) sequence of events for a mere cross/cutback? Well, simply put - overlaps suck. The average chance creation from an overlap is far smaller than that from the underlap. A cross/pass/cutback from the penalty area is worth so much more than lumping it in from afar. And very importantly (and I'm sure this was intentional from Pep), it served the Lewandowski-less team perfectly. Without Lewandowski, the team could enable the underlaps and the diagonal switches of play far better, and the cutbacks in the area work better with the enigmatic movement of Müller and the late runs of Vidal than with a (just slightly) more static target that Lewandowski provides. Whether or not Lewandowski's omittance was tactical, personal, or yellow-card related, the important thing is that the plan for the team worked perfectly to cover the weaknesses and enable the strengths without him on the pitch.
This approach also served to diminish Benfica's strength at the back as well. Lindelof and Jardel, both great in the air, we restricted to two successful aerial duels per game (pretty sure those were from corners as well), as opposed to five in the home leg. Everything about this approach worked yesterday - if not for the occasional feeling of lethargy from the team, it could've gone even better. And the way Guardiola managed to cover so many different areas of the game with this system of play still seems amazing to me.