What tactics might Alonso use at Chelsea?
Alonso is one of these coaches. While at Real Sociedad's B team, he would often start in a 4-2-3-1 or 4-3-3 shape. In his last season with Sociedad B, he had moved to 3-4-3, a formation he would become known for, using it to much success at Leverkusen.
For Alonso, these decisions are informed by players - both his own and those of the opponents. At times, he has opted for a back five in order to match an opposition back-five system, as seen in Madrid's 1-0 win against Juventus in the Club World Cup. At Leverkusen, with impressive wing-backs such as Jeremie Frimpong and Alejandro Grimaldo, Alonso had wide defenders comfortable attacking high and wide.
This made the decision to play with a back five on paper an easy one. At Madrid, the likes of Federico Valverde, Jude Bellingham, Franco Mastantuono and Vinicius Junior provided the Spaniard with a variety of players capable of thriving out wide, and therefore Madrid often played with four at the back. Out of possession, Leverkusen set up in an aggressive 5-2-3 shape that looked to step up and apply pressure, holding a high line.
Real Madrid's midfielders and attackers struggled to apply these same ideas, lacking the intensity needed. There was also the added challenge of accommodating both Mbappe and Vinicius off the ball, something Alonso tried to do in a 4-4-2 shape. Whether Alonso starts with a back four or back five, this shape is fluid.
He has shapes he looks for during certain parts of the game and asks players with the appropriate skillset to move into specific positions within that shape. For example, higher up the pitch, Alonso often looks to place five players on the last line of attack, who then take it in turn to drop deep and show for the ball. These five players can be arranged in a number of ways.