Preheat the oven to 350ºF (175ºC) and spray a 9×13-inch baking dish with cooking spray.
Season the chicken tenders evenly with 1 tsp garlic salt and 1/2 tsp Italian seasoning.
Insert one seasoned chicken tender into each uncooked manicotti shell; if a tender is too long, cut it in half and push pieces into both ends of the shell so each shell is filled.
Arrange the filled manicotti shells seam-side up in the prepared baking dish in a single layer.
In a small bowl, combine the 26-oz jar of spaghetti sauce with 1/2 cup water and stir until uniform.
Pour the sauce mixture over the manicotti, making sure the pasta is completely covered.
Cover the dish tightly with aluminum foil and bake for 1 hour (60 minutes).
Remove the foil, sprinkle 2 cups shredded mozzarella evenly over the manicotti, and return to the oven uncovered for another 5 to 10 minutes, until the cheese is melted and bubbly.
Let the dish rest a few minutes before serving.