Does the blanket have the normal belly surcingles, and is just missing one of the rear leg straps? If so, NO PROBLEM! My horse broke a leg strap once, and I had it off for about a week before I got around to sewing it back together! I just used the one leg strap and attached it diagonally from the left side to the right rear (or vice verse).
I think the worst that could happen with some intense rolling is that the blanket might flap up in the back exposing a butt cheek!