Castro bemoans disallowed goal
Jackson Martinez struck in the 57th minute on Thursday to hand Porto a narrow advantage ahead of the second leg of their last-16 tie in Naples.
Castro was understandably happy to see his side deny Napoli an away goal but he was less impressed with a decision that saw Carlos Eduardo's 19th-minute header ruled out for offside.
"We've had access to the images and the response is natural. The goal is clear. We should have won 2-0," Castro said.
Reflecting on the positives as Porto claimed a first victory in eight European games, Castro told Sport TV: "Our goal was to win without conceding a goal and it was achieved.
"We won well. Victory suits us well, but I get the feeling that we could have won 2-0."