Euro 2012 qual. - McCarthy yet to commit to Ireland
It was thought the 20-year-old had chosen the Republic of Ireland over Scotland when he was named in Giovanni Trapattoni's latest squad earlier this week.
Scotland-born McCarthy can still play for both countries as his only senior international appearance for Ireland came in a friendly last year. The former Hamilton player sparked speculation about where his allegiance lay when he withdrew from Trapattoni's squad for last month's Carling Nations Cup game against Wales citing an injury.
The Football Association of Ireland then thought they had secured the youngster after Trapattoni met McCarthy on an apparently successful recent trip to England, but Martinez has now suggested that may not be the case.
Martinez, speaking at a press conference to preview Wigan's weekend clash with Birmingham, said: "I am sure it is going to be decided shortly, but the focus goes to Birmingham for now.
"It is very unfair to try to put him under this massive pressure, it has been growing day by day. He needs to have time and space to make the right decision as to who he wants to play for at international level."