I don't think she's underweight but she is just not "fit". She's smooth, which means plenty of fat is there. If you hand walk her some distance or jog with her several times a week she will look fantastic!
Standing up in profile, even after a week her shoulder blades, backbone, and ribs nine are clearly visible from across a 20 foot room- The pictures I posted were picked carefully, she's gained 7 pound so far, her 'keel bone' and her joints had been visible before we started feeding and watering her.
The vet said it should be muscle she builds in the back, hips and shoulders but the approx weight increase would be around 10# she needs to keep the defined waist and not 'square out'.