The two teams clashed in the Emirates Cup this afternoon, with the Portuguese side running out 3-1 winners.
Porto are reported to be holding out for €40m (£34.7m) from the Serie A side, who have already reinvested a substantial amount of the money received for Edinson Cavani's sale to Paris Saint-Germain in Gonzalo Higuain.
Fonseca jokingly said that he was trying to avoid new Partenopei boss Rafael Benitez on the touchline at the Emirates Stadium.
"I don't know if you noticed, but whenever Rafa Benitez came towards me on the touchline, I would move away because I didn't want him to ask about Jackson Martinez," Football Italia quotes the 40-year-old as saying.
"As far as I am concerned, the case is closed. I am extremely confident Jackson Martinez will stay with us."
Martinez, who scored 26 goals in 20 league games last season, was left on the bench for today's game.