The Gunners were handed a relatively 'easier' task than Premier League rivals Chelsea and Manchester City, who faced Paris Saint-Germain and Barcelona - but their clash with Monaco couldn't have gone much worse for manager Arsene Wenger.
After Geoffrey Kondogbia and ex-Manchester United and Tottenham striker Dimitar Berbatov handed Monaco a 2-0 lead, Arsenal appeared to be clawing their way back into the tie when Alex Oxlade-Chamberlain curled home a shot from the edge of the box, but Yannick Ferreira-Carrasco restored Monaco's two goal lead late on to hand Arsenal a mammoth task in the second leg.
Arsenal now need to score at least three times when they visit the Stade Louis II in the second leg to stand any chance of going through - and Wenger's men are on the brink of an early exit.
On a night where Arsenal's wide players struggled, former Gunners star and current Sky Sports pundit Thierry Henry was disappointed with the movement from Danny Welbeck in particular - but gave words of encouragement for substitute Theo Walcott, who Henry believes offered width as well as the ability to cut inside.
"The only time that someone did it was when Theo Walcott came on - because he had that typical winger movement, being out and tucking in, and that's how they created a couple of chances," said Henry.
Walcott played alongside Henry at the Emirates Stadium, and even took his old shirt number (14) at Arsenal - and whilst he doesn't offer the world-class attacking threat that Henry did, he does give the Gunners blazing speed, which can help Arsenal change a game; as he almost did against Monaco.