The Blues, who are tenth in the Premier League, lost 1-0 at the Riverside Stadium in the first leg of their Carabao Cup semi-final.
While Mauricio Pochettino's men can still salvage the tie at Stamford Bridge later in January, it was symptomatic of the club's recent decline.
With hundreds of millions worth of talent on the pitch, Chelsea's strongest XI were unable to really trouble Middlesbrough's goal, and looked vulnerable on the counter-attack.
The Blues broke the Premier League transfer record twice in 2023, singing Enzo Fernandez for £108million and then Moises Caicedo for £115m, while splashing more than £1billion on players since May 2022.
But academy star Conor Gallagher has emerged as the club's highest-performing player in a seriously under-performing side.
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Still, the 23-year-old has been linked with a shock exit as co-owner Todd Boehly desperately tries to satisfy Financial Fair Play regulations with his policy of selling homegrown stars and raking in 'pure' profit.
It has led to uproar among Chelsea supporters, but one fan told the Sports Bar how the clamour for Gallagher symbolises the club's fall from grace.
"I can't even explain," caller Archie told talkSPORT on Tuesday night. "There are so many layers to this club at the minute which are just diabolical.
"I could go through the whole team. It's a complete waste of money.
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"They are not worth the money we have spent. They are not £100m players. What are we supposed to say? They are pathetic.
"They have no desire. Forget about Chelsea, if you're playing Sunday League, you're expected to run for every tackle. You're expected to run for every single ball that is a 50/50. They back out of challenges."
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He added: "We've got Conor Gallagher in midfield. He's been one of our best players this season.
"But do you really think, 15 years ago, we would be catering for Gallagher?
"When we had Michael Essien, Michael Ballack, Frank Lampard and Claude Makelele?
"He wouldn't even get in the squad."