Victor Moses's importance to Chelsea was underlined as a change to a 3-5-2 formation against Crystal Palace left Antonio Conte's sideunsettled as they dropped three vital points in the title race.
Quick-fire goals from Wilfried Zaha and Christian Benteke, after Cesc Fabregas had clipped Chelsea into an early lead,have given renewed hope to the chasing pack in the Premier League after Palaceran out 2-1 winners at Stamford Bridge.
Before Sam Allardyce's men pulled off one of the shock results of the season, Chelsea boss Conte had suffered fewer injuries than any other manager this season. He had also made the fewest changes to his starting line-up of any boss in the Premier League but he was forced into a huge one against Palace.
Moses, who has emerged as the perfect right wing-back under Conte, was missingdue to a calf strain and his absenceleft Chelsea unbalanced asPedro looked awkward as a back-up.
Pedro was focused more on his defensive duties than his attacking abilitiesand he was one of many players who were uncomfortable as Chelsea were carved open down the right-hand side for Palace's second goal.
The Spaniard's positioning, in turn, affectedN'Golo Kante and Cesar Azpilicueta, who were all over the place as Zaha and Benteke's link-up led to what proved to be the winner.
Chelsea's midfield and forward line wasn't as fluid or incisive with the switch to 3-5-2 and it may need some fine-tuning before the 3-4-3 can be abandoned again in the future.
In fact, Pedro was dragged into three different positions over the course of the 90 minutes as Conte looked to Michy Batshuayi and Willian to answer his side's problems.
Conte urged calm after the second goal, but his Chelsea players faced a situation that they hadn't seen for a long while, chasing a comeback after conceding twice. Aside from the loss of fluidity, Conte's men also lacked their usual ruthlessness in front of goal, a character trait that has defined Chelsea's incredible run in the league.
Diego Costa hasn't quite been himself since the offer from Chinese Super League club Tianjin Quanjian. He missed several sitters, includinga point-blank range strike in the 22nd minute, while his second-half header from an Eden Hazard cross was wayward.
The Chelsea striker may be on the verge of a second half of the season blip, with Hazard looking increasingly like he will be his side's go-to man in the final weeks of the season.
Tottenham have closed the gap to seven points after a 2-0 win at Burnley, while Manchester City will also be able to haul themselves back into contention, if they win at Arsenal on Sunday, but Chelsea remain the favourites and have earned the space to have a bad day in the league.
Pep Guardiola's side face Chelsea on Wednesday and, if Moses remains unavailable, then City will be provided with extra hope that they can pull off a remarkable title comeback.
Chelsea's loss to Spurs in January was quickly forgotten, while their defeats to Liverpool and Arsenal early in the season sparked a revolution, as well as a 13-match winning run.
Conte is the master of solving problems that emerge during defeats but he will either need to get Moses back or conjure something up ahead of a huge midweek fixture.
The 3-4-3 formation had turned Chelsea from Jose Mourinho's failures to the Harlem Globetrotters and it was sorely missed against Palace.