Season the chicken breasts evenly with salt, black pepper, garlic powder, and onion powder on both sides.
Place the seasoned chicken breasts in the slow cooker in a single layer.
In a medium saucepan, melt the unsalted butter over medium heat.
Whisk in the all-purpose flour and cook for 1 to 2 minutes, stirring constantly, until the mixture turns a light golden color to form a roux.
Gradually whisk in the chicken broth, ensuring no lumps form.
Bring the mixture to a simmer and cook for 3 to 5 minutes until the gravy thickens slightly.
Remove the gravy from heat and stir in the heavy cream and Worcestershire sauce until smooth.
Pour the gravy evenly over the chicken breasts in the slow cooker.
Cover and cook on low for 4 to 4.5 hours, or until the chicken is cooked through and tender.
Carefully remove the chicken breasts from the slow cooker and place on a serving platter.
Stir the gravy in the slow cooker to combine, then spoon the gravy over the chicken before serving.