Below-par Arsenal slumped to defeat last night to leave themselves with a tougher path towards Champions League glory.
With his side already qualified for the last 16 and with several key men – like captain Cesc Fabregas and striker Robin van Persie – carrying injuries, manager Arsène Wenger felt confident enough to take a calculated gamble with his team selection against a well-organised Portuguese outfit, who had never lost here to an English side a European tie.
However, the move backfired as the much-changed Gunners never really got going and were undone by goals either side of the break by Bruno Alves and Lisandro Lopez.
Arsenal will now be drawn against one of the group winners next Friday – which include Juventus, Bayern Munich and Barcelona – and so must play the second leg away from home. While the road to the Champions League in Rome is certainly not blocked for the Gunners, it has now become somewhat more treacherous.
Afterwards Wenger insisted he did not regret fielding a young side and backed his team to handle a group winner in the next round.
He said: "It will be difficult but we will be up for it and have a chance against anyone in Europe. We were quite comfortable in the first half until the goal tonight. What was disappointing was that when we tried to come back we were not capable of being dangerous, we lost balls and then we were on the verge of conceding more."
The first real chance opened up for the visitors after 25 minutes when Nicklas Bendtner, this week in bright green boots, fed Aaron Ramsey on the right edge of the Porto box. The young Welshman turned his marker and fired in a low, angled shot which Helton did well to push around the far post.
It was no surprise when the home side took the lead six minutes before the break. A corner from the left was floated into the Arsenal box, and Alves arrived in front of Abou Diaby at the far post to head past Manuel Almunia.
Arsenal stepped up a gear as they tried to get back on level terms before half-time. However, their delivery into the danger area was poor as the Porto defence stood firm.
The visitors were soon further behind in the 54th minute. Arsenal gave possession away in midfield, with Francisco Fernando picking up the ball and feeding it through to Lisandro at the edge of the box. The striker made no mistake, smashing his shot into the top right corner.
Arsenal again looked to make an immediate response, with Ramsey's neat turn and shot testing Helton.
It would have been all over for the Gunners had Cristian Rodriguez not pulled his shot wide when played in on the overlap down the left. As it was, there remained hope of a comeback and slowly Arsenal mounted some pressure.
However, Rodriguez almost got clear of Emanuel Eboué as they raced towards the Arsenal goal but the defender did enough to pressurise the striker to shoot off target. Then Lucho dragged his close-range shot wide as the Gunners were again exposed at the back.
Arsenal were awarded a free-kick just outside the box, Bendtner's shot beat the wall but the Porto keeper collected it comfortably – which just about summed up a lacklustre display from the Gunners.
Almunia summed it up, saying: "We were not in the game at any moment. We didn't pass the ball well, we had no shots on target at any time. We didn't feel good on the pitch and just weren't in the game. When we lose we have to think about what we did badly and what we have to do to improve."
FC Porto: Helton, Fucile, Rolando, Bruno Alves, Pedro Emanuel, Lucho Gonzalez (Costa, 77) Fernando, Raul Meireles, Lopez, Hulk (Guarin, 88), Rodriguez (Mariano Gonzalez, 78). Substitutes not used: Nuno, Stepanov, Lino, Sektioui.
Arsenal: Almunia, Eboue, Gallas, Silvestre, Djourou, Ramsey (Wilshere, 59), Diaby (Gibbs, 59), Song Billong (Randall, 78), Denilson, Vela, Bendtner. Substitutes not used: Fabianski, Hoyte, Merida, Simpson.
Referee: K Vassaras (Greece)
Results: Dynamo Kiev 1 Arsenal 1; Porto 3 Fenerbahce 1; Arsenal 4 Porto 0; Fenerbahce 0 Dynamo Kiev 0; Fenerbahce 2 Arsenal 5; Porto 0 Dynamo Kiev 1; Arsenal 0 Fenerbahce 0; Dynamo Kiev 1 Porto 2; Arsenal 1 Dynamo Kiev 0; Fenerbahce 1 Porto 2; Dynamo Kiev 1 Fenerbahce 0; Porto 2 Arsenal 0.