Jackson Martinez's 57th-minute strike secured a slender advantage for Porto ahead of next week's second leg in Italy.
While Napoli had opportunities of their own, Reina believed the Portuguese champions deserved their triumph on Thursday.
"In the second half we had a few chances, though I have to say it is a bad result, but a fair one," Reina is quoted as telling Mediaset.
“Porto are a team who move the ball around well and in the first half we waited for them so we could go on the counter.
"When we had the ball, though, we did not use it well and tended to lose possession too quickly."
Napoli retain high hopes of reaching the quarter-finals and can take heart from a 100 per cent home record in Europe this season.
Rafael Benitez's side won all three of their UEFA Champions League group games at Stadio San Paolo en route to an unfortunate exit from the competition.
The Italian outfit then beat Swansea City 3-1 last month to book a tie with Porto, who had gone seven European fixtures without victory prior to Thursday's success.
"We need a different Napoli in the second leg," Reina added.
"The fans will be by our side and we'll give our best to go forward."
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