Sometimes it all comes down to one piece of genius.
In an all-Premier League tie, Tottenham Hotspur progressed to the last 16 by beating Watford 4-2 on penalties after the game, played in Milton Keynes, ended 2-2.
Jurgen Klopp made a total of eight changes and it showed, with a second-string defence never looking particularly authoritative.
As for Liverpool, they can get revenge for the defeat at the weekend, with the two giants meeting again (again!) in the Premier League on Saturday afternoon.
Liverpool look far less convincing with Alberto Moreno at left-back than with Scotland captain Andy Robertson in that position, while they simply have no adequate replacement for Virgil van Dijk at centre-half.
"That's outstanding football, and now (he is) doing it in a very interrupted pre-season, without a lot of players involved - Eden (Hazard), (Olivier) Giroud - and starting the season, and you see immediately the impact".
Danny Welbeck happy to wait for Premier League start at Arsenal
And also [he will have] more chances to find the first XI for the next matches with a performance like today. It was straightfoward for Arsenal up to that point - but Brentford made them sweat after the restart.
The Belgium winger went past two Liverpool players around the halfway line before sending the ball through Robert Firmino's legs to Cesar Azpilicueta on the right flank.
The Reds face a double-header against the Blues, first playing them on Wednesday in the cup, and then on Saturday in the Premier League.
Watching with interest on Saturday will be champions Manchester City who are also unbeaten with five wins and a draw. Klopp has yet to cover all of his bases at Liverpool, although he is getting much closer to getting that job done.
The Carabao Cup fourth-round draw takes place on Saturday night. They block players, my players.
Hazard has already scored six goals this season - it took him until November 22 to reach that tally last term - and his new manager believes he is capable of scoring at least 40 for the campaign under his guidance.
And it did not disappoint at all, as both Jurgen Klopp and Maurizio Sarri sent out attacking teams, despite resting a few key players.