Rub 2 tablespoons of the dry rub onto both sides of the filleted chicken breasts. Grill or cook in a greased skillet until cooked throughout (or until the chicken reaches an internal temperature of 165 degrees).
In a small mixing bowl, whisk together the remaining dry rub and the pineapple juice, mix until a thick sauce forms. Brush generously over the cooked chicken.
To assemble the sandwiches, place half of the lettuce on the bottom half of the bun, then add the chicken, followed by the pineapple, red onion, remaining lettuce and the top of the bun. Enjoy hot.