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."
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