Season the chicken breasts evenly with salt, black pepper, garlic powder, and onion powder on both sides.
Place the seasoned chicken breasts into the slow cooker.
In a medium saucepan, melt the unsalted butter over medium heat.
Whisk in the all-purpose flour and cook, whisking constantly, for 2 to 3 minutes until the mixture is golden and bubbly, forming a roux.
Gradually whisk in the chicken broth, continuing to whisk until the mixture is smooth.
Bring the mixture to a simmer and cook for 3 to 4 minutes until the gravy thickens.
Remove the saucepan from heat and stir in the heavy cream and Worcestershire sauce until fully combined.
Pour the gravy 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 dish.
Stir the gravy in the slow cooker to combine and adjust seasoning with salt and pepper if needed.
Pour the gravy over the chicken and serve immediately.