The clubs meet at the Estadio do Dragao on Thursday in the first game of their two-legged clash, in what will be interim boss Castro's second match in temporary charge following Paolo Fonseca's departure.
Fonseca left after a run of four games without a win in all competitions, before Castro begun his time at the helm with Saturday's 4-1 win against Arouca.
And ahead of Porto's meeting with Rafael Benitez's Serie A high-flyers, Castro is predicting a close encounter against a side who have not tasted defeat in their last eight matches.
"This round with Napoli will be very tight and I do not think that there is a favourite at this time," he said.
"Without saying too much, we will have to be balanced in defence and attack. Both teams will play without opening up, so I expect a very balanced game.
"I feel that the players are very confident and motivated, but we are also aware of the value of our opponents."
Having dropped out of the UEFA Champions League, Porto negotiated their first Europa League test thanks to a late goal from Nabil Ghilas - his equaliser against Eintracht Frankfurt sending the Portuguese champions through on away goals.
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