What Causes Accumulation Of Fluid And Blood Post Knee Cartilage Surgery?

my husband had knee cartilage surgery (cadaver graft). This surgery was 2 weeks ago and he was doing fine but he did develop some fluid and blood collection. He had it drained but the doctor mentioned a hematoma that isn t liquified. My husband is in worse pain since the drainage occurred and the area is purple and swollen. His main concern is the amount of pain and whether it s normal.