Excellent job getting the images online. It helped a lot.
Although fractures are out of my scope of practice as a chiropractor... judging from the images you posted, it seems likely that the tibia should heal fine.
It is possible that the fibula may need screws and/or a plate due to the diagonal orientation. You may consider getting a second opinion on surgery from a different ortho surgeon. The casting may be enough to heal the fracture without surgery, as long as it does not displace in the meantime. Good Luck!
Antonio Marotta, D.C.