But it was the visitors who looked the more sprightly in the early stages, and Rangers goalkeeper Allan McGregor held well after Flood's replacement, David Robertson, burst into the box to meet a long throw-in and stab the ball towards goal from 14 yards.
Another Mexican substitute, the 37-year-old Cuauhtemoc Blanco, sealed the victory from the penalty spot after a third replacement, Pablo Barrera, had been felled in the box.