Chelsea boss Mauricio Pochettino bemoaned a lack of cutting edge after after his team lost the first leg of their English League Cup semi-final 1-0 to Championship side Middlesbrough at the Riverside.
Hayden Hackney scored the only goal on 38 minutes, condemning Chelsea to a fifth defeat in their last six away games.
The Blues must turn the tie around when the sides meet again at Stamford Bridge on January 23 to avoid heaping further pressure on Pochettino.
"We were the better side, had clear chances, but we didn't score and weren't clinical," said Pochettino."
Boro boss Michael Carrick hailed the "very special" victory against a team of Chelsea's quality.
Chelsea should have at least been level at the break but Cole Palmer twice failed to hit the target with glorious chances.
In the second half, Chelsea pegged the home side back but could not find a way through.
