Hayden Hackney scored the game’s sole goal, sliding in an Isaiah Jones cross in the first half.

Cole Palmer squandered three good chances as Chelsea, which had 18 shots but only five on goal, failed to score for the sixth time this season.

“It is difficult to talk about our performance,” said Chelsea manager Mauricio Pochettino on BBC Radio 5 Live.

“We generated some opportunities, but we weren’t clinical enough. We created some chances in the first half of the season, but we did not win because we were not clinical enough; we must continue to press.

Chelsea, currently ranked tenth, has wasted 40 huge opportunities in the Premier League this season, the second-highest total in the top flight, after only Liverpool (41).

They intend to sign a forward in the next two transfer windows, but they are unlikely to pay the large sum required to sign Brentford’s Ivan Toney or Victor Osimhen from Napoli.

Boro, who have lost six home Championship games this season, may now dream of reaching the final after winning the event 20 years ago.

They entered the game with 12 first-team players out and lost attacker Emmanuel Latte Lath and left wing-back Alex Bangura to injury in the first 20 minutes.

“The way they stuck together was very special, and it makes me proud to see that,” Middlesbrough manager Michael Carrick told Sky Sports.

“We wanted to exhibit a little of everything, and I’m buzzing. I understand what it means to everyone; it was amazing for me to see so many people happy, and it’s quite remarkable.