Frank Lampard’s side suffered a narrow 1-0 loss at the hands of Spanish club Valencia at Stamford Bridge in London in Group H.
Rodrigo scored the only goal of the match between the Blues - who won the Europa League last season - and Los Che in the 74th minute.
Penalty disappointment
Chelsea, though, could have at least shared the spoils had former Everton attacking midfielder Ross Barkley not failed to convert a penalty.
The Premier League club were awarded a spot kick after a VAR decision adjudged that Daniel Wass had blocked Fikayo Tomori’s header.
Barkley, who had come on as substitute only eight minutes earlier, insisted on taking the spot kick despite Jorginho and Willian being on the pitch.
The 25-year-old England international attacking midfielder, though, failed to convert it, as Chelsea tasted defeat on their return to the Champions League.
Stats
Over the course of the 90 minutes at Stamford Bridge on Tuesday evening, hosts Chelsea had 59% of the possession, took 22 shots of which six were on target, and earned 10 corners.
Visitors Valencia had 41% of the possession, took eight shots of which two were on target, and earned two corners.
Bouncing back
Chelsea will be looking to bounce back from the defeat to Valencia on Sunday evening when they take on Liverpool at Stamford Bridge in the Premier League.
The Blues are back in action in the Champions League on October 2 when they lock horns with Lille away from home in France.