The interim boss feels that the Portuguese side should have scored more at the Dragao but he is taking great encouragement out of their 3-3 draw in Frankfurt in the round of 32
Luis Castro remains frustrated by Porto's profligacy in the first leg of their Europa League last-16 clash with Napoli - but is still confident that they will progress to the quarter-finals.
The Portuguese champions had the better of last week's meeting between the two sides at the Dragao but only ended up with a Jackson Martinez goal to show for their efforts.
Consequently, Castro freely admits that Porto are in a perilous position ahead of the second leg, yet he is taking great encouragement out of the team's dramatic 3-3 second-leg draw with Eintracht Frankfurt in the previous round.
"There was a lot of pessimism when we played away in Germany after a 2-2 draw at home," Castro told reporters on the eve of the game at the San Paolo.
"Now we come into a second leg on the back of a positive result, although it wasn't the right result as we could have scored more goals but only won 1-0.
"We are optimistic, although we have a few injuries I am sure we will go out onto the pitch and give our maximum to get to the next round.
"That is our objective."