This looks like a shaved area of fur where the IV would have been placed for the procedure. It's difficult to tell the position from the photo, but if this spot is near the crook of the elbow, then that is definitely what it is. Otherwise, the skin looks healthy and the area of hair loss is too neat to be due to any bad causes.
I think you are fine to wait and see if it starts growing hair and doesn't have any signs of infection (flakey skin, redness, scabs, pus). If all looks OK, then the area should be back to normal is about 3-5 weeks.