If you are nervous about her rolling, there are little triangle positioners that you can buy to keep her on her back. Rolled-up blankets also work well. They say not to have blankets in the crib because of the possibility of suffocation, but if you place the blanket no higher than chest level, that should work. It always worked for us.
Generally, when a baby can roll, that means they have enough neck and core strength to be able to move, and the risk of SIDS decreases dramatically. I understand, though, wanting to do everything possible to keep your little one safe.