Hey HorsieLover, that's one huge hematoma. Walka has had 2, though a little smaller then that one.
Didn't drain, not always successful. Vet recommended hot packing with moist heat,(towel in hot water well wrung out) for 10 - 15 mins 2 x's a day. After a week started using DSMO and that worked like a charm. Really helped the body reabsorb the blood pool.
Good luck, and let us know what your vet recommends to do.
Wanted to add, reabsorbed beautifully, no ripples or anything to indicate he ever had any.