Chelsea fought hard against Arsenal at Stamford Bridge, but Liam Rosenior's side have it all to do in the return leg at the Emirates three weeks from now.
The 3-2 scoreline probably reflected the flow of the contest, with fans sure to be pleased by Rosenior's sharp substitutions, Alejandro Garnacho bagging twice off the bench to keep the tie in the balance.
Surprise, surprise, the Gunners took the lead through an early corner, but Chelsea should have done better. In fact, for all the gritty guile on show, the Blues left something to be desired across a number of areas.
What went wrong for Chelsea vs Arsenal
Rosenior is clearly a confident manager, and his track record thus far bespeaks a shrewd and attractive tactical style. However, he has much to learn in the Stamford Bridge hot seat.
Chelsea head coach Liam Rosenior
Chelsea were unconvincing on the ball, gifting Mikel Arteta's side possession numerous times in dangerous areas. However, any blunders made by Wesley Fofana and co paled in comparison to Robert Sanchez between the sticks.
The Spanish shot-stopper should have done better to claim Declan Rice's free-kick flapping as it arced over him and found the head of Ben White.
Sanchez also allowed the ball to wriggle past him after the interval, giving Viktor Gyokeres space from close range to tap in and double the visitors' lead moments into the second half. Rosenior's mid-match regulating swiftly came undone.
But Chelsea weren't only blighted by mistakes. A number of key personnel were missing on Wednesday evening, and one stand-in blew his big chance in the middle of the pitch.
Chelsea talent blew his big chance
Moises Caicedo's absence was keenly felt, and though Andrey Santos is an immense talent, the 21-year-old flattered to deceive against England's standout side this season.
Gabriel Martinelli and Andrey Santos-1
Santos is a strong and combative midfielder with technical qualities in spades. However, he was tentative and unsure against Arsenal, weak when he needed to be strong. This was a game that demanded a controlled and convincing performance from midfield, and Santos let Rosenior down in that regard.
Penning their post-match thoughts, London World handed Santos a 5/10 match score, bettering only the 3/10 score of Sanchez.
Andrey Santos vs Arsenal
Match stats
Minutes played
Goals + Assists
Touches
Accurate passes
Chances created
Dribbles
Ball recoveries
Tackles won
Duels won
Fouls
Dribbled past
Data via Sofascore
Given that Santos lost 66% of his duels and struggled in key moments - he should have done better for Martin Zubimendi's goal - this was a poor performance from the young lad, and it will have done little to stop the Chelsea faithful from pining for the suspended Caicedo.
We know what he can do. For Strasbourg in Ligue 1 last season (under Rosenior's wing), the all-action Brazilian chalked up 43 appearances and scored 11 goals, playing with maturity and menace. However, he hasn't translated that form over to England just yet.
Sanchez had a tough evening, but he still pulled off a wondrous save against Mikel Merino in the second half. His compatriot's volley would have marked game over, had it found its way into the net.
Santos, however, showed he still has improvements to make if he wants to nail down a regular starting berth in a Chelsea side chasing success of the highest calibre.
With Caicedo coming back, it's a given that he will be dropped.
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