Chelsea suffered an FA Cup fourth-round scare on Saturday when they had to come from behind and needed extra time and a penalty save to beat League One Plymouth with goals from Spaniards Cesar Azpilicueta and Marcos Alonso.
Plymouth shocked the Premier League side when defender Macaulay Gillesphey nodded the ball home from a free kick in the eighth minute, sparking wild celebration and a shower of green balloons from the travelling fans.
Chelsea coach Thomas Tuchel had tested positive for COVID-19 and missed the game while his expensively assembled squad looked rusty and heavy-footed after a fortnight's break.
Chelsea hit the woodwork three times in the first half, twice through Mateo Kovacic and again through Callum Hudson-Odoi but Plymouth's well drilled defence kept the European champions at bay until the 41st minute when quality told.
Captain Azpilicueta scored with a neat backheel from a low Mason Mount cross.
Plymouth absorbed waves of Chelsea attacks in the second half and extra time and last-ditch defending as well as inspired goalkeeping from Mike Cooper kept the hosts out until the 16th minute of extra time.
Alonso broke the deadlock when he got on the end of a pass from Kai Havertz and swept the ball in and with five minutes to go Kepa Arrizabalaga saved a penalty from Ryan Hardie.
Chelsea remain unsure whether Tuchel will be able to travel to Abu Dhabi for any part of the Blues' Club World Cup campaign.
The Blues will jet out to the United Arab Emirates on Saturday night without the German, with their Club World Cup semi-final set for Wednesday.
"First of all we do not exactly know right now what the situation is with Thomas, when he will be back," said assistant coach Arno Michels.
"He has to follow the Government rules, and is in self-isolation. So we are looking forward for the next days. We were in ongoing communication with Thomas during the match.
"He knows about the game of course, and we stayed in touch with him the whole game. Everything was clear what we wanted to do. We stayed in touch the whole time and all was good."