Chelsea’s formation and positioning was different with and without the ball.
With the ball, Kovačić would usually drop to the side of Kanté to collect the ball (ahead of West Ham’s midfielders) and play forward, through dribbling and passing, where he was very influential and effective. To give more time and space on the ball for Chelsea’s defenders (central defenders and Kanté 3v2 vs Fornals and Antonio) Kovačić could move higher up on the line of West Ham’s midfielders, where Chelsea had to circulate the ball to move Antonio away from the ball and create the spare man.
Barkley stayed higher up and played between lines (behind West Ham’s midfield line), but was fairly ineffective in this role since West Ham’s backline were very tight and deep, and he was unable to break behind or carry the ball forward into space as he had been doing effectively. Chelsea played few crosses into the box, depriving Barkley of yet another avenue where he had been making a difference.
From the wings, Willian stayed wide (sometimes rotating with Barkley to move inside with the ball) while Pulisic played on the inside between the lines and behind West Ham’s midfield line. Pulisic’s movements and positioning created problems for West Ham, since he would draw Fredericks narrow and inside (leaving space wide for a number of switches to Alonso overlapping), and Fredericks had great difficulty getting close or passing Pulisic on to another defender.
Without the ball, Chelsea set up in a 4-3-3, with Pulisic wide, Kanté deep and Kovačić and Barkley higher up — because West Ham played with two deep midfielders and one high, so it was natural to match them. However, when pushing up on one side of the pitch, Chelsea could still shift into a 4-2-3-1 with the winger from the far side moving inside to cover one West Ham midfielder with Barkley on the other (and Abraham between central defenders). Without the ball, Kovačić was once again excellent (second game in a row) when pressing and anticipating the passes to make interceptions or close down the receiving player quickly. Chelsea’s best midfielder during the first half.
West Ham didn’t seek to press high and win the ball high up as Chelsea’s recent opponents have, While Chelsea have had a lot of problems when building through pressure like that, they also hadn’t conceded many clear chances as a result due to their excellent reactions to losing the ball. So by dropping off and looking to recover the ball from deeper instead, West Ham would open up more spaces to attack from midfield and deep positions. Antonio would make runs into the channels during the first half that Chelsea’s defenders controlled well, however these spaces in wide areas would be more significant later on in the game.
West Ham also relied upon their power in the air from set pieces. Antonio taking long throw-ins provided a platform to get the ball in the box with numbers to attack the ball, along with corners. The latter saw the ball going into Chelsea’s net twice during the first half. Chelsea couldn’t compete with the matchups inside the box and defending set pieces has been a constant weakness of the team throughout the season.
Pulisic was Chelsea’s main offensive threat in the final third through his movements and exceptional footwork when dribbling with the ball in tight spaces, as well as arriving in the right places for shots on goal. Without surprise he won the penalty, from which Chelsea took the lead, before West Ham equalised.
West Ham made a good start to the second half, and took advantage of their best period with the ball in the game to take the lead. Unlike recent games when Chelsea waited to press forward passes from midfield and launch dangerous counters, Chelsea would instead push up and press in rash moments, which opened the spaces between lines for West Ham to play through and move into the final third (instead of making it harder for the player on the ball). The same problem would occur for West Ham’s winner later on in the game.
Chelsea made the first change of the game following the goal, replacing Kovačić (our best midfielder during the first half) with Mount. Mount would play between lines more and around the box, and even get into the box as well, but this also meant that more of Chelsea’s attacks would get stuck with slow ball possession on the wings without being able to get the ball into the box.
The next set two changes made a much more of positive difference, with Giroud replacing Abraham and Loftus-Cheek going on for Barkley. Giroud in particular would create much more of a threat inside the box where he was always getting into positions that were difficult for West Ham defenders to control. Pulisic continued to be Chelsea’s main creative threat and he again won the free kick, from which Willian would go on to score to level the game.
However, as the half entered its closing stages, Chelsea opened up more and more with both fullbacks high and wide to cross the ball into the box, and committing all of three midfielders forward towards the edge of the box. This left spaces to exploit when Went Ham won the ball, and Antonio staying wide (combining with Fornals to lose Christensen) would provide the outlet needed to break Chelsea’s backline and create a 3-v-2 inside Chelsea’s half. Antonio found Yarmolenko breaking forward on the far side 1-v-1 against Rüdiger dropping back (Fornals also breaking free behind Christensen to Antonio’s left) where he would take the ball into the box and cut inside onto his left foot to shoot and score the winner for West Ham.
West Ham approached the game differently than Chelsea’s recent opponents, playing for moments and to their strengths. At the same time, Chelsea continued to have significant problems defending corners (which West Ham took advantage of), and didn’t create many chances to score. Chelsea only began to threaten West Ham’s box through crosses to Giroud after going behind in the match during the second half, and in over-committing to the box for the win in the final moments of the game, would leave open space for West Ham to counter and score a late winner.