Firstly, here is how the draw will work.
It was a depressing evening for both clubs at Stamford Bridge on Tuesday as neither got what they wanted from their final group fixture.
There was also something to cheer for Nigerian striker Henry Onyekuru as his Belgian club exited Europe with their heads high as they beat Celtic 1-0 in Glasgow.
Roma pipped Antonio Conte's men to top spot in Group C by beating Qarabag 1-0, while the Premier League champions only salvaged a point through Stefan Savic's own goal after Saul Niguez had put Atletico on course for the win they needed to stand any hope of staying in the competition.
Catalan giants Barcelona finished top of Group D ahead of Italian champions Juventus.
Arsenal's quest to win the Europa League for the first time in their history has become a significantly tougher prospect after the introduction of several prominent teams into the Round of 32 draw.
There are still some issues to be resolved in Wednesday night's Champions League games.
Liverpool won their group after hammering Spartak Moscow 7-0 at Anfield tonight.
Elsewhere Manchester City have already won Group F. However, Napoli and Shakhtar Donetsk are still fighting it out for the other place in the knockout stages of the competition.
Group H runners-up Real Madrid notched 17 as Cristiano Ronaldo made history by becoming the first player to score in all six first-phase games, while Harry Kane hit six for the section's winners Tottenham.
But with the January transfer window opening in less than four weeks, a host of Premier League scouts will have been more occupied with casting their eyes over European football's brightest talents.