Actually, Millar was a first baseman before he came to Boston. I remember watching him play first base, and a some third base, for the Portland Sea Dogs when they were a Marlins' farm team back in the late '90s. He played 446 minor league games at first, 133 at third, and only 30 in the outfield. He was trained as a corner infielder.
In the majors with the Marlins, he never played more than 30 games in the outfield in a season. If it hadn't been for the acquisition of first baseman Derek Lee by the Marlins, Millar would probably not have played any games in the outfield. He was never primarily an outfielder.