On the eve of the tie, Mikel Arteta challenged his players to prove they belong on this stage but they laboured for large parts of the game and failed to register a shot on target throughout a largely drab affair, losing 1-0 at the death courtesy of a stunning Galeno strike.
With the second leg at the Emirates Stadium on March 12, Arteta now knows his side must win on home soil to reach the business end of the Champions League on their long-awaited return.
Arsenal, more than anyone, know there are no easy games at this stage of Europe’s elite club competition – having fallen at this hurdle seven years in a row under Arsene Wenger.
This is their first time back at this level since the last of those in 2017 and it proved much more challenging than the recent Premier League thrashings of West Ham and Burnley.
Porto currently sit third in the Primeira Liga, off the pace of their big rivals Benfica and Sporting Lisbon, but they were a match for the Gunners, frustrating the visitors for much of the evening.
Declan Rice was walking a tightrope for almost the entirety of the contest after he was booked for a late challenge on Galeno with just 67 seconds on the clock.
Arsenal were not at their recent, slick best and were very fortunate not to fall behind just before the midway point of the first half as Galeno wasted two great chances in quick succession.
After a cross dropped to him in the box, with the Brazilian fizzing a shot off the far post and then somehow turned the rebound wide from six yards – the home fans flummoxed as they celebrated along with the music that greets a goal for the hosts inside the Estadio do Dragao.
The let-off failed to rouse Arsenal from something of a slumber, but they did finally have their first shot on the evening after 33 minutes and should have taken the lead themselves soon after only for William Saliba to head a Bukayo Saka corner wide.
Porto were still the more likely to open the scoring, though, as Evanilson drew a low save out of David Raya with what proved to be the only shot on target from either side until the late winner.
Kai Havertz, who scored the winning goal in a Champions League final in this stadium for Chelsea, was next to miss the target with a header from a Saka corner.
Set-pieces certainly seemed Arsenal’s best way to goal and Leandro Trossard will feel he should have done better as he peeled off to the back post from a corner early in the second half, only to fire Rice’s centre harmlessly over.
There were no other opportunities of note until the dying embers, when Galeno collected the ball and brilliantly bent a finish over the out-stretched Raya as the whole Porto bench burst onto the pitch in celebration.
For Arsenal, they return to their Premier League title challenge knowing their backs are against the wall in their quest to progress in Europe.
