Dinamo Zagreb condemned Chelsea to the worst possible start to their Champions League campaign with a 1-0 victory at the Stadion Maksimir, Zagreb on Tuesday.
Pierre-Emerick Aubameyang made his Chelsea debut, wearing a mask after suffering a broken jaw following an armed robbery at his home in Barcelona.
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Dinamo took a suprise lead 13 minutes in as Chelsea were split open by a two-pass counter-attacking move. Talisman Mislav Orsic latched onto Bruno Petkovic's header and beat Wesley Fofana before placing a delicate finish past Kepa Arrizabalaga.
Dinamo, who were under a significant amount of Chelsea pressure before the opening goal, grew into the game and became the assertive force in the first half.
Arijan Ademi was next to trouble Kepa, forcing the Spain goalkeeper into a good low save from a well-struck effort just outside the box.
Aubameyang tapped home four minutes into the second half, but his debut goal was ruled out as Mateo Kovacic drifted offside in the build-up.
Stefan Ristovski went close to adding a second for the impressive Dinamo when his effort was pushed onto the crossbar by Kepa as the hosts' forays into Chelsea's final third continued to prove more fruitful than Thomas Tuchel's side's attacking exploits.
"It's an underperformance from us. We don't finish our half-chances. We conceded a counter-attack that is far too easy," Tuchel told BT Sport.
"There's too much to analyse, I am part of this. We are clearly not there, where we need to be and where we can be.
"At the moment, everything is missing. I will not talk about individuals, we play as a team, we lose as a team.
"I need to find a solution for the next match. It's not enough."
In search of the illusive equaliser, Chelsea sustained heavy pressure on Dinamo in the closing stages. Robert Ljubicic blocked well before a series of chances saw Reece James hit the post and Jorginho have an effort saved. Dinamo, who have conceded in every league game this season, held on and are the early trailblazers in Group E.