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."
Castro was installed as Porto's interim coach until the end of the season following the resignation of Paulo Fonseca on March 5.