Martinez has developed an individual style at Everton, with Barry and McCarthy playing as a defensive pair. Everton attack through the fullbacks Coleman and Baines, with Barry or McCarthy falling back to cover when one of them goes upfield.
McArthur would perhaps have been a more natural replacement for Barry than McCarthy. But Rosler has played him as an attacking midfielder and McArthur has shown he can fill that role too.
McArthur could provide cover for either Barry or McCarthy and I’m surprised, but not unhappy, that Martinez hasn’t come back for him too.