After a stunning performance midweek against Leicester in the FA Cup, it wasn't necessarily a surprise to see Heung-Min Son in the XI for Saturday's Premier League trip to Selhurst Park. It was a surprise, however, to see that he was in for Erik Lamela. Though the Argentine has struggled for goals lately, his importance to the team is still undeniable. Lamela drifts laterally and links up play nicely, but also is able to keep the game stretched a bit by staying wide when asked to do so. Plus he's tireless worker when asked to press the ball which makes him an ideal Pochettino player.

By dropping Lamela and keeping Dele Alli in to play alongside Son, Pochettino made a clear announcement of how Spurs would attack Palace. Once again, Christian Eriksen would drop off to form a midfield three while Son and Alli would flank Harry Kane up top. (We've written about Eriksen's evolving role and the fluidity of Spurs' shape here at CFC.) In other words, Spurs were going to let their best passer drop into deeper midfield roles and then rely on aggressive vertical running from both wide attackers to help them get in behind the Palace defense. It was a good idea in theory, but in practice it didn't work for two reasons.

First, one key to making the 4-3-3 work is having a midfielder who can surge forward out of midfield into the vacated advanced central area that exists between midfield and the attacking three. This line-breaking player is especially important when a team sits deep and then presses hard on certain cues as Palace did at the weekend. If no players are ever breaking into those spaces between the lines, then the attack is likely to be disjointed and inconsistent because all three attacking players will become isolated.

When you look at the midfield trio of Eriksen, Mousa Dembele, and Dier, there isn't a single player in that group who is going to look to make aggressive vertical runs either with or without the ball. Dembele and Eriksen both drift laterally when out of possession, looking to receive the ball played into their feet. Dier, meanwhile, sits deep as the holder. When in possession, Eriksen is going to look to play a quick pass rather than dribble at the defense. Dembele will obviously run at the defense, but he only seldom makes the sort of Yaya-Toure-like runs into the box that would help make the 4-3-3 system more effective. It's more typical for him to dribble into an advanced central area and then move the ball either through the channels to the wings or play it out wide to a fullback.

The net result of all this is that Tottenham often had two quite distinct layers of play in midfield—one in a deeper position consisting of Eriksen and Dembele and one further forward consisting of Alli, Kane, and Son. But between those two layers there was nothing but space or Palace defenders. This problem was apparent almost from the beginning of the game. Here is a shot from two minutes into the match:

And the problem continued to plague the Spurs attack throughout the first half. Here is another shot much later in the match. It is slightly better because Dembele is beginning to push forward and Alli is starting to come deeper, so the players aren't as isolated, but there still isn't much going on here in terms of link-up play:

The second problem is simpler: Spurs were not passing the ball well. Part of this is attributable to Palace's defensive work, but a lot of it was just sloppiness:

The problem persisted well into the second half as well. This sequence came moments before Dier was withdrawn. In it you'll see Eriksen stop short as he's making a vertical run and you'll then see Dier misplay a pass, thereby ending the attack:

The net result is that Spurs were mostly unable to break down the Palace defense in the first half. There was no lateral drifter in the attacking third since Eriksen was playing deeper and Lamela was on the bench. There was also no midfielder making the kind of vertical runs that help to open up space in the attacking third.

This led to the disjointed, sloppy attack that characterized much of the first half. The team began to correct for this late in the first half and especially after the Vertonghen own goal with Eriksen and Dembele pushing slightly forward and Alli dropping deeper, but it was only providing mixed results. And far too often you saw what we see in the GIF immediately above: Eriksen or Dembele dribbling for a bit, stopping, looking for a pass, and being forced to play the ball backward.

The Chadli substitution introduced the player Spurs needed to link their attack—and removed their sloppiest passer on the day.

In one sense the Chadli substitution was simply addition by subtraction. Eric Dier had often been the guilty party when Tottenham turned the ball over in midfield so removing that liability could only help the team's passing—provided that Dembele could handle the defensive responsibilities typically left to Dier.

The bigger change, however, came from specifically introducing a player like Chadli. Like Lamela, Chadli is a versatile attacking forward who can play anywhere across the front three. He doesn't have the superlative qualities of a Lamela or Eriksen, but he's a safe, tidy player who can play in a variety of ways and has more of an eye for goal than the Argentine—and recall that Spurs were chasing the game when Pochettino made the change. He also links up the midfield and attack with his lateral movement in a way that no one else on the field for Spurs at the time does.

Within one minute of coming on, you can already see Chadli checking his vertical run and instead drifting in laterally to present for the ball. You can see him in the top left corner of the frame here. Eriksen doesn't pass to him, but you can see the difference Chadli makes. He's making it possible for Spurs to retain possession in the attacking third in order to put the Palace defense under greater pressure and he gives the team the option to make that one extra pass that can create a real scoring chance:

Moments later Chadli did this to set up what might have been a chance for Kane, but the striker was flagged for offside in the buildup:

This sequence didn't lead to a goal for Spurs, but it shows the difference Chadli made. Whereas Alli and Son mostly made vertical runs, in the above clip we see Chadli coming a bit deeper and showing for the ball and then making a wonderful touch to create the scoring chance for Spurs.

Moments later, Chadli was (again) at the heart of things as Spurs equalized through Harry Kane:

Though the goal doesn't come from Chadli's lateral movement in the attacking third, it's still attributable to the substitution not only because Chadli provided the assist but because Chadli's linking ablity in the attacking third is what allowed Tottenham to build the sustained pressure that led both to the opener above and to Dele Alli's winner 20 minutes later.

Speaking of Alli, it wasn't just Chadli's introduction that changed things in the Spurs attack. It was also the shifting of Dele Alli out of the front three and into the midfield two with Dembele. Thanks to this shift, Spurs not only had Chadli to link the attack in the attacking third, they also had Alli making his vertical runs from a deeper, more central position–which created new difficulties for the Palace defense. Here is a screen capture from a typical passage of play in the second half after the Chadli substitution. Spurs have just turned the ball over and Palace is about to attempt a counter, though it won't lead to anything. The key thing to see here is the number of players Spurs are getting forward:

Note that Chadli has drifted in. Rose has made an overlapping run to provide width. Eriksen is in a more advanced role. Kane is up top. Son is advanced, as is Alli. The team now has six players pushing into that attacking third area whereas for much of the first half they had three.

By introducing Chadli and dropping Alli to midfield, Pochettino changed the shape of the team which allowed them to move the ball more easily in the attacking third and get numbers forward, both of which led directly to Tottenham's comeback victory.

Conclusion

The most encouraging thing about this win for Spurs isn't necessarily the fact that Nacer Chadli once again looked like a useful player or that Dele Alli scored a golazo to win it. It's that Mauricio Pochettino made the change that won the game for Spurs. The ability to make a change like that to turn around a poor performance is something that neither Andre Villas-Boas or Tim Sherwood could do on a consistent basis. Even Harry Redknapp only did it occasionally, as when his halftime move to bring on Jermain Defoe turned around the 2010-11 Emirates North London Derby and helped Spurs overcome a 2-0 halftime deficit to win 3-2.

Given that the most consistent knock on Pochettino has always been his lack of a Plan B when Plan A isn't working, the fact that he could make such an unusual substitution and turn the game so effectively is quite encouraging. Certainly, Dele Alli's golazo won the three points for Tottenham, but it was Mauricio Pochettino's substitution that changed the game and made Alli's golazo possible.