Chelsea missed a host of chances as Championship side Middlesbrough earned a slender advantage in the first leg of their Carabao Cup semi-final at a boisterous Riverside Stadium.
Hayden Hackney scored the only goal, opening up his body to slide home an Isaiah Jones cross in the first half.
Cole Palmer missed three presentable opportunities as Chelsea, who had 18 shots, but only five on target, failed to score for the sixth time this season.
“It is difficult to talk about our performance,” Chelsea boss Mauricio Pochettino told BBC Radio 5 Live.
“We created some chances, but we were not clinical enough. This first half of the season, we have created some chances, but we have not won because we were not clinical enough – we must keep pushing.”
Chelsea, who sit 10th, have missed 40 big chances – as defined by statistical analysts Opta – in the Premier League so far this season, the second-highest total in the top flight, behind Liverpool (41).
They are looking to bring in a forward in the next two transfer windows but it is unlikely they will be spending the huge money needed to land Brentford’s Ivan Toney or Victor Osimhen from Napoli.