Porto snatched a shock last-gasp 1-0 victory over Arsenal as Galeno netted a sublime effort deep in stoppage time in their Champions League last-16 tie on Wednesday.
A drab clash was deadlocked until the fifth minute of added time when the Brazilian winger curled an unstoppable effort past David Raya to send the Estadio Do Dragao crowd wild.
Arsenal, who had scored 21 goals in their last five Premier League games, were strangely subdued in attack and failed to register an effort on target throughout a cagey contest.
They defended solidly enough but may live to regret their lack of urgency as Galeno gave Porto the edge ahead of the second leg in London on March 12.
Arsenal are seeking to reach the quarter-finals for the first time since 2010 with their last seven Champions League campaigns all ending in the last 16.
"We had to manage the game better when you can't win," Arsenal manager Mikel Arteta said. "We lacked threat and aggression in the final third and we need to tweak a few things. We can do better.
"If you want to be in the quarter-finals you have to beat your opponent and this is what we will have to do at the Emirates now."
Reassuringly for Arsenal, Porto have a terrible record away against English clubs and have never scored at Arsenal, conceding 11 in three visits.
In Wednesday night's other tie, Napoli fought back to hold visitors Barcelona to a dour 1-1 draw,as Victor Osimhen cancelled out Robert Lewandowski's opener.
Last year's Spanish and Italian champions, both struggling this season, played out a poor match in which both sides were toothless and lacking attacking flair.
Third in the LaLiga standings eight points behind leaders Real Madrid, Barcelona dominated the first half against a disorganised Napoli who barely managed to get out of their own half.
Barca were playing their first Champions League knockout match for three years while Napoli were led by new coach Francesco Calzona with their Serie A title defence in tatters as they languish ninth in the domestic standings.