What Causes Reduction In Haemoglobin Levels Post Femur Bone Grafting?

My husband a paraplegic for 30 yrs broke his femur 3 days ago. He has been bedridden for the past 5 months due to grafts on both buttocks. In the ER, his hemoglobin was 8.2 so 2 units of blood were given. During surgery to screw a rod in his leg, an additional 2 units were given. Yesterday his hemoglobin was 7.3 so he received an additional 2 units. Today his number is 8.6. Is this common or should I be concerned.