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."
Porto manager Vitor Pereira said a 'win-to-nil' was exactly what he was chasing in their home leg.
Despite Pellegrini's gripes at the goal that was allowed, Pereira believes his side could have scored more to move closer to sewing up a spot in the last eight.
"Our first goal was to win, our second goal was not to concede. But after the great quality match that we played from the first to the last minute, I think we deserved a second goal," he said.
"I'm not completely satisfied in that we couldn't get a goal to make it 2-0; that would have been a fairer reflection of what happened on the pitch.
"That's the Porto I want - a team that puts on pressure, that wants to control the game, that wants to score. I'm just sorry that we could not score the second.
"The difference between the Champions League and other competitions is that an error, a moment where we are not well positioned, could be a moment when the opponent scores, so it was very important not to concede here at home. We were a united and consistent team."
The second leg will be played at Malaga's La Rosaleda on March 13.