Malaga coach Manuel Pellegrini says Porto scored from an offside position to win 1-0 in first leg of the Champions League round of 16.
La Liga club Malaga went down by a solitary goal in the first leg at Estadio do Dragao on Tuesday, with midfielder Joao Moutinho finding the net in the 56th minute.
Pellegrini was critical of the decision to award the only goal of a tight game, insisting the scorer was offside.
"Porto didn't really hurt us and the goal was offside," he told reporters.
"I thought from the beginning that Moutinho was offside."
Pellegrini was also unhappy with a lacklustre showing from his own players, especially going forward, but he remains hopeful of turning the tie around in the second leg.
"We were not a good Malaga because Porto pressed us well," he said.
"We didn't create much space. Playing a game this badly, we didn't create many chances. I hope that, at home, we can reverse the situation.
"We can turn the scoreboard at home if we can play as (we normally do). We played badly going forward."
The second leg will be played at Malaga's La Rosaleda home on March 13.