Arsenal should have been out of sight by halftime in the north London derby, so how did Spurs end up with a draw?

While teams operate with base formations, there is a collective responsibility for defending and for attacking.

Just because you’re picked to play from the left, it doesn’t mean you don’t go back and help your fullback, or tuck in and help your midfield when the game situation calls for it.

When new personnel or tactics are implemented, it can take time for everyone to adjust.

The first 45 minutes of Sunday’s North London Derby featured Spurs players struggling to adapt to unfamiliar roles in an untried system. They were given a thorough examination by a profligate Arsenal side.

Pochettino’s decision to come to the Emirates with a back three was an intriguing one, with this group of Arsenal players having enjoyed recent success against similar systems, most notably defeating Basel 2-0 and dispatching Liverpool 4-1 in 2015.

One characteristic of Arsenal’s dominance in these matches was their use of the halfspace, the area between the central zone of the pitch and the flank.

Arsenal would exploit gaps and create situational overloads with Mesut Özil drifting into these zones, with support from a fullback and central midfielder, creating a 4 vs 2 or at worst 4 vs 3 situation, in which they could play triangles around their opponents and work the ball into the box. Arsenal have used this ploy to good effect this season following the emergence of Alex Iwobi and did so again against Spurs.

After 15 minutes on Sunday, Arsenal figured out that Walker and Rose were looking to stay as wide as possible. With Eriksen and Son floating behind the returning Harry Kane, this left an awful lot of ground for Wanyama and Dembele to cover in the Spurs engine room. As such, both Iwobi and Walcott could drift inside, looking to attack the halfspace.

If Walker or Rose went with Arsenal’s nominal wide men, it would present an opening for Bellerin or Monreal to overlap.

However, with Bellerin and Monreal so eager to get forward to attack space, Arsenal were leaving Koscielny and Mustafi exposed at the back with less than ideal collective cover.

Sunday was the first time the German had looked anything less than assured in an Arsenal shirt, especially when Spurs countered. He was bailed out a number of times by the imperious Monreal, who demonstrated that he’s still Arsenal’s best left-back.

While Spurs carried a threat on the counter, their first half-ball circulation and progression of play was poor. At no point did you worry they’d put together a 12 pass move that would disrupt Arsenal’s settled lines. Due to a lack of a deep build, they looked to go long from back to front, but dealing with this was one area in which Arsenal’s centrebacks excelled on the day.

Before the match, there was talk of Tottenham’s much-vaunted pressing game disrupting Arsenal’s rhythm, as Pochettino sides have done in all past encounters with Arsenal. However, Tottenham’s tactical tweaks temporarily robbed them of the requisite familiarity to press as aggressively and effectively as they have previously done.

Arsenal also attempted to evade Tottenham’s press by playing direct balls to runners on the counter, looking to get 3 vs 3 against the re-jigged Spurs backline, exploiting the uncertainty and indecision created by the unfamiliar roles for Walker and Rose.

Arsenal played balls into the areas of the pitch that would usually be occupied by traditional fullbacks, which would drag Tottenham’s widest centreback out into unfamiliar territory.

While Vertonghen has experience of playing left back and was equipped to deal with being dragged into these wide areas, Eric Dier struggled, with most of Arsenal’s chances in the first half coming from his side, most notably for Iwobi, Özil, and Walcott.

Had Arsenal’s ball release and timing of runs been better, they could have been in on goal several more times and been out of sight at half time, but were rightly thwarted by the offside flag.

For swathes of the first half, Arsenal outnumbered Spurs in midfield and were thus able to either completely bypass or play around their press. Just as Chelsea’s flawed centrebacks have benefited from strength in numbers and having their individual deficiencies masked since their move to a back three, Francis Coquelin thrived in this numerically superior setting in the first half, breaking up play and quickly shifting the ball on to Granit Xhaka, who excelled in his first North London Derby.

However, after half time, Tottenham’s midfield clicked.

Eriksen started dropping deeper, the wing backs offered more support, and most crucially Dembele moved deeper into a position where he could receive the ball off his centrebacks and build play for his team.

It was the powerful Belgian whose dribbling got Spurs back into the game.

After evading Özil on half way and running unchallenged alongside an unresponsive Coquelin for 25 yards, the powerful Belgian was fouled in the box by Koscielny. A Harry Kane penalty later, Spurs were back in a game that Arsenal should have already put to bed.

After the equaliser, both sides upped the tempo and the game became more stretched and error-strewn.

Spurs looked to Dembele for inspiration, while Arsenal continued to look to work the halfspaces and play diagonal balls in from wide.

It was a fundamentally sound plan, which just didn’t come off as a result of poor finishing.