After scoring 29 goals in 38 appearances in all competitions last season, Colombia international Martinez has been strongly linked with a move away from the Portuguese champions, with Rafa Benitez's side believed to be at the front of the queue for his signature.
Fonseca, however, is confident he will keep the 26-year-old at Estadio Dragao this summer - and insisted they would cope with his loss even if Martinez decided to leave.
"We are never ready to lose great players like Jackson Martinez," Fonseca said after Porto's 1-0 defeat to Galatasaray in the Emirates Cup on Saturday.
"But it seems it is not going to happen now, he will stay here, this is what I believe anyway.
"If it does happen, Porto always responds very well when great players leave the club. Only recently we lost Joao Moutinho and James Rodriguez but Porto will remain a strong team with good players."
The former Chiaspas forward was named among the substitutes for Porto's clash with Napoli on Sunday.