Islam Slimani's first-half winner means Porto have just a point to their names after two games in Group G.
Nuno Espirito Santo bemoaned a poor start by his Porto side as they lost 1-0 to Leicester City in Champions League action on Tuesday.
Islam Slimani scored the only goal of the game at the King Power Stadium in the 25th minute, keeping the Premier League champions top of Group G.
Porto, meanwhile, have only a solitary point to their names in a group that also includes Copenhagen and Club Brugge, while they have now failed to win in 17 games away to English clubs.
"It is not easy to come to the Premier League champions," Nuno told reporters.
"We didn't start well. It was not the way we planned it, even though I think we had the first and second chance.
"After a couple of mistakes, we allowed Leicester to play how they like to play."
Defender Felipe said Porto must quickly turn things around in Europe, with their next game in the competition seeing them travel to Belgium to take on struggling Club Brugge.
"We have to think about the next game and fix everything," he told the club's official website.1 comment