I had a partial knee replacement 34 days ago. No on told me NOT to put a pillow under my knee. I have slept every night with one. I have 138 degree range of motion and a negative 3 degree of extension. The PT people are pretty amazed and my surgeon acts like it is normal. I just ordered a between the leg pillow to take the pressure off of my lower back where I have disk problems. Everyone is different. Good luck and speedy recovery.