Arsenal manager Mikel Arteta said his team lacked the cutting edge as Galeno's stoppage-time goal earned Porto a 1-0 win in the first leg of their Champions League last-16 clash.
The Gunners were appearing in a first knockout match in Europe's top tier competition for seven years, but struggled to get past Porto's defence before Galeno's brilliance in the 94th minute.
The defeat ended Arsenal's eight-game unbeaten run against Portuguese opposition and leaves them with plenty to do in the return leg at home on March 12.
"We lacked threat. We lacked aggression, especially when we had the ball in the final third," Arteta told TNT Sports. "So we will tweak a few things to attack better. We can do better."
"Obviously I am very disappointed the way we gave the game away at the end," he added.
"Not managing that situation well enough. You get punished in the Champions League. If you cannot win it, you don't lose it.
"We really dominated the game but we lacked purpose, especially in the first half. You need to have much more aggression, you need to break lines, to play forward and generate much more threat on that backline.
"We will learn from it. Now it is clear, it is half-time. If you want to be in the quarter-finals you have to beat your opponent and that will be the purpose and the plan."
Asked if his team were naive to fall to defeat in such a way, Arteta added: "Well it's only the last ball, so if in 94 minutes they haven't had any naivety other than that one.
"I think it's a bit cruel to judge it. But it's true that it has had a big impact on the result. A lot of other things they did for the first time here were very good. When you give the ball away three times in that area - you cannot do it."
Arsenal last made it to the Champions League quarter-finals in 2010, when they beat Porto in the last 16.
"I think it's a team that's very well-organised defensively and they break your rhythm all the time," said Arteta.
"We want to be in the quarter-finals, you have to beat your opponent and this is what we have to do at the Emirates."
Arsenal failed to register a single shot on target as their recent goal spree came to a shuddering halt.
Meanwhile, Barcelona manager Xavi Hernandez said he was satisfied with the point from their 1-1 draw with Napoli in their Champions League last-16 match but he could not help feeling the first leg tie had been there for the taking.
Barca spurned several opportunities to score before Robert Lewandowski netted the opener in the 60th minute. But they failed to add to their tally and were pegged back when Victor Osimhen equalised from Napoli's only shot on target 15 minutes later.
"I think we had a great match, both offensively and defensively," Xavi told reporters.
"The team did some things to deserve to win, but this is the Champions League.
"But I leave here satisfied. Satisfied, but a bit bitter because from my point of view, we could have won."
Barcelona host Napoli, last year's Serie A champions, in the return leg on March 12. They return to La Liga action on Saturday with the visit of Getafe.
