Chelsea coach Carlo Ancelotti will have been unconcerned to see a weakened team beaten by Newcastle United in the League Cup but Saturday's Premier League loss at Manchester City was another matter.
After five thumping wins in their opening five league games against modest opposition, it was a case of Chelsea failing their first serious test of the season and Ancelotti warned that unless the team remember the basics there will be more defeats to come.
"I had an easy message for my players (after the Manchester City game)," said Ancelotti. "It was 'When we play as individuals we lose, when we play as a team we win'. This is it - the story of the match."
Chelsea will be missing the suspended Didier Drogba for the Group F game at Stamford Bridge, meaning the Ivory Coast striker will have to wait for a chance to face the club where he made his name.
Drogba, who joined Chelsea from Marseille in 2004 for a then club-record 24 million pounds, has scored five goals already this season and created as many again.
Chelsea did not miss him in their opening 4-1 win away to Zilina but the clash with French champions Marseille is likely to be a far trickier proposition for Ancelotti's double-winners.
Victory though would put Chelsea in control of the group and clear the path to the last 16.
Marseille desperately need something from the game after losing to Spartak Moscow at home in their opening match in the group but Deschamps knows exactly how difficult it will be at his former club.
The former France World Cup winner spent a season at Chelsea in 1999/2000 long before Roman Abramovich's money transformed it into one of the continent's big hitters.
"When I joined from Juventus it was a culture shock," Deschamps, 41, told the Marseille website. "It was not the same Chelsea as it is today. It was a good English club but now they are huge."
Deschamps was coach of Monaco when they beat Chelsea to reach the Champions League final in 2004 but still holds the London club in huge affection, apart from the visitors' changing room.
"Stamford Bridge breathes football, it's a beautiful stadium apart from the visitors' locker room," he said. "That's a horror. It's a storage room; there is no room."
Deschamps will hope to cramp Chelsea's style on Tuesday and his side warmed up with a 2-1 home win against Sochaux in Ligue 1 at the weekend in which central defender Souleymane Diawara returned from injury.
"I played there (at Stamford Bridge) twice and I was not that impressed by the atmosphere," the former Bolton Wanderers player told Marseille's website.
Chelsea are still without Frank Lampard and Yossi Benayoun but Ashley Cole will return to left back after being rested for the trip to Slovakia in the opening round of matches.
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