Arsenal were startled when Galeno scored in stoppage time to give Porto the lead in the first leg of their Champions League last-16 clash.
The Brazilian winger took a few moves to set himself up before curling a beautiful finish into the net beyond David Raya’s desperation full-stretch dive.
Sergio Conceicao’s side should have been up at halftime after Galeno fired a shot off the post and somehow missed the rebound.
That was a tremendous letdown for Arsenal, but Galeno made no mistakes in the game’s final four minutes.
Nico Gonzalez’s thunderous effort had earlier gone over the crossbar as the home team attempted to open the score in the first half.
The Gunners dominated possession but had no shots on goal in the game, with William Saliba and Kai Havertz hitting wide from corners.
The second leg will be held at Emirates Stadium on March 12, with Arsenal aiming to reach the quarter-finals for the first time since 2009-10.
Arsenal had numerous set-piece possibilities but missed a precise finish. In one of such circumstances, Declan Rice’s corner was met on the volley by Leandro Trossard, who fired over the crossbar.