After his team's loss to the Portuguese outfit, the newly-capped Spain international believes the opposition's winner should have been disallowed.
The former Sporting man gave his side the lead in the 56th minute, with Porto going on to win 1-0, but Isco believes the decision by the official to grant the goal was the wrong call.
"The goal was illegal and it shouldn't have counted," the 20-year-old told Goal.com. "But that stuff happens."
The youngster then criticized the performance of his team, but also gave credit to the opposition for successfully shutting down the game of the Spaniards.
"It has been a bad game for us. We couldn't enjoy the ball and they made it very complicated for us at every moment," he said. "There is still a lot to play for and we must trust we can turn this around before our fans. We leave sad but we still believe we can advance."
The second leg of the tie will be played in Malaga on March 3. Prior to that, Manuel Pellegrini's men will resume Liga action with a trip to Betis on Feb 24.