Chelsea must come from behind in the return leg at Stamford Bridge after losing the first leg of the Carabao Cup semi-final.
Cole Palmer was guilty of two appalling misses during a first half in which Hayden Hackney put Championship side Middlesbrough ahead.
After Boro’s Jonny Howson presented the ball to Palmer, he fired wide from near the edge of the penalty area.
And after Chelsea had fallen behind, he should have equalised shortly before half-time when the loose ball dropped to him after keeper Tom Glover was unable to Enzo Fernanez’s shot, but Palmer contrived to shoot over.
It was not a good night either for Levi Colwill.
With Ian Maatsen in Germany to complete a loan move to Borussia Dortmund, central defender Colwill was deployed as a makeshift left-back and struggled badly.
Boro’s goal came when Isaiah Jones got away from Colwill and crossed to Hackney, who applied the finish.
Chelsea: Petrovic; Gusto (Gilchrist 90), Silva, Disasi, Colwill; Caicedo, Fernandez (Broja 63), Madueke (Mudryk 63), Gallagher, Sterling; Palmer.
Subs not used: Bergstrom, Bettinelli, Golding, Williams, Gilchrist, Washington.