When Mauricio Pochettino trotted out a 3-4-2-1 in the December 28th fixture against Watford many fans were confused by the move. Some didn't even realize that he was doing it initially because the personnel was basically no different than what he uses when playing 4-2-3-1. All that had changed was Eric Dier dropped deeper than normal to play between the center backs with the fullbacks pushed further forward and Tom Carroll partnering Mousa Dembele in midfield behind a front three of Dele Alli, Erik Lamela, and Harry Kane.
Some initially thought this was a sign of Pochettino mimicking his former boss and mentor Marcelo Bielsa, an understandable read given Pochettino's close relationship to Bielsa but one that is badly mistaken. Bielsa plays a three man back line because he uses a basically insane form of man-marking with a sweeper playing behind the defense. Pochettino's use of the three man defense was much simpler than that: When facing two striker systems, it makes sense to play three at the back so that you can have one defender responsible for each striker with a third defender free to play the ball. It's the same reasoning for playing a two man defense in the vast majority of games—you want one more central defender than the opposition has strikers.
Besides, Pochettino's own take on pressing is actually more conservative than Bielsa's. Bielsa uses a strict man-marking system and presses everywhere. Pochettino uses a more zonal pressing system and has his team press on specific cues. (To be clear, saying Pochettino is more conservative than Bielsa does not mean Pochettino is actually conservative; it just means he isn't as crazy as the man many in the football world know as "el loco.")
That said, when we move past the misleading comparisons with Bielsa we can see that Pochettino is actually still quite versatile in how he sets his team up. Though notionally a 4-2-3-1 team, Pochettino's Spurs can actually shape-shift into several different formations with relative ease.
Sometimes Spurs play in a 4-4-2.
The much-discussed two banks of four approach to defending is very much en vogue today with Atletico manager Diego Simeone being the foremost proponent of it. In Simeone's system he uses two extremely narrow banks of four stacked right on top of each other with two center forwards tucked inside buzzing around near midfield waiting to launch a counter.
Pochettino has not used this system too often at Spurs, although he did play something eerily similar to it in his first ever North London Derby when a struggling Spurs side went to the Emirates and were unlucky to draw 1-1. But you still see him use it in limited cases even this season after his pressing system has been fully installed at Spurs.
Recall that Pochettino only presses the ball on very specific cues. In cases when the opposition has the ball in the attacking third and is not triggering the press, Spurs will drop into the 4-4-2 by default. This season that has meant Erik Lamela and Christian Eriksen dropping to the wings alongside the midfield duo with Dele Alli and Harry Kane pushed just ahead of the two banks of four. This screen capture from the win against Norwich shows how it looks in practice:
In the above image Harry Kane is further up field and is not in the frame. But the two lines of four are clear with Dele Alli pushed just ahead between Mousa Dembele and Lamela.
What this approach does for the team is it makes them solid in defense as they face a controlled attacking opportunity from the opposition. Ideally, of course, Spurs will only spend short amounts of time in this system because what they want to be doing is pressing the ball and ending the chance before it can even begin to take shape.
But in a situation like the one above, pressing the ball would be very foolish because the Norwich player in possession, Russell Martin, can easily recycle possession backwards, play a ball down the wing for the wide man, or even whip a cross into the box and test the Spurs defenders or goalkeeper Hugo Lloris. In that case, the team's best chance at defending the situation effectively is to retain their shape and intercept whatever pass Martin tries to play.
Sometimes Spurs also shift into a 4-3-3.
That first change is not at all unusual, of course. In addition to Simeone's Atletico, many teams will drop off into a 4-4-2 when put in a spot like the one above. Jurgen Klopp's Liverpool will drop off in a similar fashion, as would Jose Mourinho's Chelsea. Arsenal can also shift into a defensive 4-4-2 at times.
The shift to a 4-3-3 is a bit more interesting. To make clear what is happening, we'll need to look at how Spurs line up on paper and compare it to what happens when they shift to 4-3-3. Here is the base 4-2-3-1 we have seen for most of the year:
This shape doesn't really capture how Spurs like to play though. To begin, Lamela and Eriksen often end up swapping wings, although the above is what we have seen more and more often in recent weeks. Second, Dembele usually pushes further forward than Dier, although when Dier is out Dembele shifts into the defensive role. Third, Kane will often drift wide to receive the ball. Thus the above shape can easily transition into either of the shapes shown below. The first is the more conventional 4-3-3, the second is the more radical one which has shown up more often since the emergence of Dele Alli.
Here is the conventional 4-3-3:
In the above situation, Spurs rely quite heavily on Kyle Walker to get up and down the right wing, but as Walker has great pace and is a willing runner, that doesn't really create any major problems. It also helps that Eric Dier, who can play right back, can shift right to cut off attacks if Walker is further forward.
That said, the more significant version of 4-3-3 that Spurs play is something more like this:
Note that in this set up both Kane and Lamela are capable of playing on either wing. The key point is the use of Dele Alli as a false nine. This is often how Spurs ended up functioning against Norwich. The screen capture below shows the basic shape, although Dier has actually pushed forward as we was closing on the ball before Norwich successfully recycled possession.
However, a few seconds after this image was taken Norwich played the ball forward, which created a transition sequence when they were unable to control the long pass. Eventually the ball bubbled out to midfield where Erik Lamela won it, triggering a counter opportunity for Spurs. Lamela passed it quickly to Alli who charged at the Canaries defense with Kane to his right and Lamela to his left. This was the result:
Obviously full marks should go to Kane for hitting a great shot to score from a very tight angle. But the structure that leads to the chance is Pochettino. By dropping Eriksen deeper to partner Dembele and dropping Dier even deeper to hold, it created a numbers mismatch for Norwich in their own attacking third which, in turn, led to the transition sequence ending with Lamela picking up the ball.
It's possible to make too much of Pep Guardiola's famous quote about Pochettino's Espanyol emphasizing the similarities in their approach to the game. But one key way in which Guardiola and Pochettino are very similar is the way they expect their players to be comfortable in multiple roles across the field. Literally every outfield player in Pochettino's preferred XI has played multiple positions in their top flight career. The result of this versatility in each individual player is that it is easy for Pochettino to set up a fluid side that can morph into multiple attacking and defensive shapes over the course of a 90 minute match.
So the move to a 3-4-2-1 against Watford wasn't actually that surprising. In the first place, it was simply following common footballing wisdom about how to set up against a team that plays two strikers. But second, and more important, it is entirely feasible for Pochettino to set up his same core XI in three or four different shapes depending on the need of the moment. It is this flexibility, amongst other things, that makes Pochettino's Tottenham so difficult to match up against and helps to explain how a team that is almost certainly less talented than Redknapp's Spurs are playing at a higher level than they ever did.