A crushing defeat at Brighton ensured that it was far from a happy homecoming for Graham Potter and Marc Cucurella.
An early period of pressure paid off for the hosts, as Trossard showed a great bit of composure to take the ball past Kepa and provide the fired-up home crowd with an early lead.
Soon after, Ruben Loftus-Cheek's unfortunate touch diverted a Solly March corner into the back of Chelsea's net. Similarly, Trevor Chalobah managed to repeat Loftus-Cheek's 'heroics', just before half-time. Brighton's ruthless first-half performance saw The Seagulls head into the break in a 3-0 lead.
Kai Havertz pulled a goal back, swiftly after the restart and gave Chelsea hope, but they were unable to put the hosts under any significant pressure, from that point on.
Pascal Groß gave Brighton a fourth goal in stoppage time and set the final scoreline - a 4-1 win for the hosts.
Chelsea remain in fifth place in the table. They may now be overtaken by Manchester United, by the end of tomorrow.
A first win for Roberto de Zerbi as manager of Brighton lifts them up to seventh place. With a trip to Wolves and the visit of Aston Villa remaining for Brighton to negotiate, prior to the World Cup break, they may feel they are capable of maintaining a push for the European places.
Story of the Match
Following their successful midweek trip to Austria, Potter made two changes to his Chelsea team. Loftus-Cheek and Mason Mount came into the 11, replacing Jorginho and Pierre-Emerick Aubameyang.
Similarly, Potter's replacement at Brighton - de Zerbi, also made two changes to his side. From their defeat at Manchester City, Kaoru Mitoma and Pervis Estupinan took the places of Joel Veltman and Danny Welbeck - both missed out, due to injuries.
The match began at a frantic pace, with the hosts immediately putting their former manager's team on the back foot.
Within the first few minutes, Thiago Silva's poor clearance picked out Moises Caicedo and led to an eventual effort from Trossard. The Belgian lobbed the ball over Kepa, but the retreating Silva pulled off a brilliant clearance off the line, to redirect the ball for a corner and rectify his mistake.
From that resulting corner, Silva was forced into making another headed clearance off the line. Estupinan's side-footed volley was well struck and equally well dealt with by the experienced centre-back.
Having got through a few very serious early warning signs, the panic in the visitors' defence showed no signs of averting.
Mitoma breezed past the passive Chelsea resistance and played a simple ball across to Trossard - inside the Chelsea box. This time around, Silva was unable to produce some goal-line heroics. Trossard showed brilliant composure to take the ball around Kepa and wait for the perfect moment to slot the ball past the covering Chalobah and Cucurella.
The majority of the home crowd had clearly not forgiven Cucurella for his summer move to Chelsea. The full-back had to endure the consistent jeers from the home crowd, whenever he was on the ball.
His mood would not have been improved, as Brighton managed to extend their early lead.
Just before the quarter of an hour mark, Solly March's in-swinging corner struck Loftus-Cheek, who was standing unaware, at the heart of Chelsea's box. The contact off his knee led to the ball deflecting into Kepa's goal, off of a touch from the post.
Soon after, Conor Gallagher wasted a guilt-edge chance to get Chelsea back in the game. With plenty of time and space, a mere few yards away from Robert Sanchez, most of the crowd would have been expecting to see the back of the net ripple. However, Gallagher's effort was too central and gave Sanchez the chance to pull off a great save. Christian Pulisic's subsequent rebound was also disappointing - he blazed his volley wide.
The visitors seemed set on putting right their poor start. Sterling's cross from the left side was met by Gallagher - his glancing header was acrobatically kept out by Sanchez. Havertz' effort, from inside of the box, called the keeper into action, again.
At around the half-hour point, a good bit of play around the Chelsea box from March and Trossard led to the ball scrappily falling to Caicedo. After the midfielder's instinctive effort, from a tight angle and with Kepa closing down the space Caicedo had available, the ball clipped the outside of the near post.
The 0-2 deficit did not seem like a critical one for Chelsea. The end-to-end nature of the game meant that the visitors could still maintain hopes of getting back into the contest.
However, as their search for a goal before half-time went on, Brighton were able to strike a hurtful third blow.
A brilliant through ball from deep set Estupinan away, in behind the Chelsea defence, down the left side. The wing-back galloped his way into the visitors' box and put in a low ball across goal. As Trevor Chalobah attempted to block the cross, his faint touch diverted the ball into the back of the net.
Unsurprisingly, that third goal seemed to take the wind out of Chelsea's sails. Brighton put the visitors under further pressure, prior to the interval and went into the break with a huge lead.
However, Chelsea were able to lift themselves, during the interval. It was a significantly better start to the second half for them, than it was in the first. Potter changed his team's system to a four-man defence at half-time and they swiftly earned a much-needed goal.
Gallagher's cross from the right side was perfectly placed. Havertz met the ball, with a header placed directly at Sanchez - the keeper will feel disappointed that he was unable to keep the effort out of his goal.
A deflected cross from Mount forced Sanchez to react swiftly; it seemed as if the drama was only just starting on the south coast, given Chelsea's energetic start to the second half.
However, the period of play following that quick start did not live up to that expectation. Cucurella got the opportunity to hear from his old club's crowd one last time, as he was substituted in the 64th minute.
Edouard Mendy had replaced Kepa at half-time - the Spanish keeper suffered a knock during the first half. Mendy's poor footwork was on show, as his unsuccessful pass picked out Mitoma and gave Trossard an opportunity to chip the keeper; the substitute was able to avoid further embarrassment.
The energetic Julio Enciso was introduced for the hosts, halfway through the second period. Within just over ten minutes, he was able to pick up a yellow card, for a rough challenge on Mateo Kovacic and have a couple of efforts at Mendy's goal.
Havertz blazed a first-time effort over the bar and that was Chelsea's last real chance of the game. They were unable to cause the hosts too many issues, after their lively start to the second half.
As the game entered added time, Groß fired in a rebound, after Enciso's effort and restored Brighton's half-time three goal lead.
Player of the match - Pervis Estupinan
It is fitting that Cucurella's replacement in this Brighton team picks up the award, in this game.
The acquisition of the wing-back appears to be another brilliant bit of business by a very efficiently-run Brighton.
Estupinan was an effective presence, both in and out of possession. His energetic run down the left side earned the hosts their third goal.
The deal to sell Cucurella and sign Estupinan has made a healthy bit of profit for the club. However, it does not seem as if it has weakened their team.