In the zone: What we learned from the Champions League group stage

Teams building from the back, switches of play, and crosses as a key source of goals. These are some of the things to look out for when the UEFA Champions League resumes on Tuesday.

Ahead of the start of the round of 16, UEFA's technical observers have looked back on the group stage which took place between September and November to identify tactical trends and topics for deeper analysis as the knockout rounds unfold between now and the final in Istanbul on 10 June.

Playing from the back

When it came to building attacks, the group stage featured a common strategy of teams using possession to draw the opposition on to them, inviting pressure and then looking to beat the first line of the press.

This was evident from Matchday 1 when Shakhtar Donetsk earned an unexpected 4-1 victory at Leipzig by purposely encouraging their hosts to press and then picking the right moments to play forward quickly, exploiting the wide areas to impressive effect.

Frank de Boer, one of UEFA's team of observers, saw most teams trying to build from the back during the group stage – for example with two central defenders and one central midfielder.

It is an approach which requires patience and involves some risk and UEFA's analysts will look closely at how successfully teams are carrying this out in the knockout stage ahead.

One thing that is clear is the goalkeeper now plays a central role that was unthinkable a decade ago, with an acceptance among fans and media that teams will take risks as they play out this way – and increased confidence in the footballing abilities of the men between the posts. "The goalkeepers have to have so much quality with the ball at their feet," affirmed De Boer.

Role of goalkeeper explained

One increasingly common trend, highlighted by UEFA observers, is for one of the centre-backs to take the goal kicks and pass to their goalkeeper as the midfield pivots drop deep and wingers go high.

One UEFA observer, Packie Bonner, offered an explanation for why defenders play the ball to their goalkeeper in this way.

"The reason for the centre-halves taking the goal kick is that the goalkeeper can take up a different position, he can go a bit deeper, away to the side of the goal and it gives him that extra couple of seconds to make a decision," he said. "He has his head up when the ball comes to him so he can see the picture better."

Bonner added: "Players are now comfortable with allowing teams to press them to a point where they have a solution to get out as the goalkeeper will always be there to provide support so they drag people into positions and allow that to happen. That's a change from the past where people were really nervous about it."

If the majority of UEFA Champions League teams look to build from the back, one question that arose among the observers is whether there are times when it pays to take a different approach. Manchester City are one team with the capacity to provide an element of surprise with Ederson's long kicks, for example.

"We forget that the direct solution can be a good solution," said Willy Sagnol.

Progressing up the pitch

As teams progress upfield, there are other ploys that UEFA's observers identified. In the middle third, it was common to see rotations to open up space. The best teams will usually have options in the wide areas – from their wing-backs notably – and Roberto Martínez saw Chelsea profit in this way in their 3-0 home win over AC Milan, noting that "switches of play in the middle third hurt Milan along with quick play involving both wing-backs".

Another team who caught the eye were holders Real Madrid who excelled at switches of play. Bonner remarked: "Toni Kroos and Luka Modrić suck you into a space and then they can exploit free spaces on the opposite sides."

Advancing up the pitch, one recurring feature of final-third progressions were overloads on one side, often followed by a change of direction.

This was integral to Benfica's success in finishing top of Group H. In their 6-1 win at Maccabi Haifa on Matchday 6, for example, they targeted the left side of their hosts' defence with overloads as winger David Neres came inside to create space for his team-mates down that flanks.

Similarly, De Boer cited the example of the wingers at his old club Ajax, explaining how they would receive support from full-backs making overlapping runs or a midfielder breaking beyond the back line. If the opposition defender was following the runner, this allowed the winger to make a run inside and either shoot or cross to the far post. Ajax ended the group stage with the highest accuracy of passes into the attacking third (71.6%).

One vs one threat

While overloads are advantageous in giving teams a numerical advantage on one side of the pitch, the importance of one vs one ability cannot be overlooked.

"Nowadays because spaces are so small for individuals, the individual actions of the wingers are so important to create something," said De Boer.

To offer an example, when Benfica beat Juventus in Lisbon, Thomas Schaaf hailed the Portuguese side's strength in "creating one-against-one situations again and again and asserting themselves in duels".

Meanwhile, Group A winners Napoli were the group stage's most potent team, with 20 goals, thanks in no small part to the speed of their wide attackers as they broke very quickly, getting the ball behind opposition back lines.

As the observer's report noted of their home win over Ajax: "The goals Napoli scored were the result of effective play in wide areas and the willingness to penetrate behind the defence."

The power of the cross

Overall, there were 304 goals scored in 96 games in the group stage, at a ratio of 3.17 per match. The two main sources were identified as a) crosses and b) teams taking advantage of a lack of balance on the part of the opposition, picking the right moment to break beyond the defensive line.

To focus on crosses, these were the source of more than one-third (37%) of the goals scored.

The UEFA observers discerned two types of cross in particular. First, there was the cut-back which made up 40% of goals scored from crosses, and then there were crosses when the man delivering the ball came inside and played it between the centre-back and full-back on the opposite side.

It is interesting to note that according to UEFA's data, the average number of players in the box when crosses were struck were two attackers and four defenders.

This raises a point about the anticipation and positioning of defenders. Observers suggested that while they were able to get back into their zones, defenders were less good at actually marking strikers and detecting their movements off the shoulder. They also argued that full-backs defending the far post must track better the runs of opposition wingers coming inside.

Setting up against the counter

Last season's technical report explained how in possession Bayern München would set up in a 1-3-2-5 and the Group C winners – the only team to take maximum points in the group stage – have not changed that approach this season.

When attacking they effectively split into two groups of five with four defenders and defensive midfielder Joshua Kimmich forming one group and a second group comprising two wingers and three central attacking players. With the full-backs stepping up alongside Kimmich, that makes for a 1-2-3-5.

According to Martínez, the inverted full-back has a pivotal role out of possession, helping to protect a team's shape when they lose the ball. Other teams tried other ways to ensure they would not be caught out on turnovers: Group B winners Porto, for instance, attacked with one full-back and one central midfielder behind the ball. And the evidence suggests that these structural ploys are paying off as only 13% of goals came from counterattacking in the group stage.