Assuming the pot-bound rose is a grafted plant, my first thought would be that the rose I'd just purchased was from a previous year's inventory.

I would loosen and spread the root mass to the degree possible, and then prune the roots -- probably quite a bit if the nursery had pruned the top growth before putting the potted rose out for sale again. Roots are fairly resilient parts of the plant, and 'damage' to them generally spurs healthy new root production.

When I get a pot bound plant I tickle the roots loose from the sides of the root ball or slice the roots with a knife in half a dozen places around it. This helps them grow into the surrounding soil when planted. Just rubbing the root ball with your hand will loosen the roots.

I've sliced off the wad of roots. I've loosened the roots. I've planted the roots. It doesn't seem to make the slightest difference.

Pruning roots is no different than pruning top growth... I remove all visible bound roots, sometimes tearing, sometimes cutting and then plant as usual... new ones will grow soon enough to take their place...

The reason the pot is root bound is because it was in a pot that was too small... make sure to put it in a bigger pot so that the problem doesn't happen again...

I whack 'em with an ax sometimes. If you have a solid mass of roots it has to be pruned off. I routinely use the shovel, when planting, to prune off the bottom inch or two or four of the rootball to remove wadded up roots, then slice down the sides, or shave them off entirely. The only roots that pick up water and nutrients are the tiny young rootlets; all that other stuff is just mechanical support.

That's for extreme cases. If it's a mild case, then you can just tickle them loose, or make a few scores down the sides and along the bottom. Whatever roots come loose, spread out in the hole.

I once read an article in a nursery trade journal where the author recommended pruning all roots of all woody nursery stock back to just a few inches, so the plant can grow new roots in its new soil, rather than have it sitting in container mix or field clay while planted in a different soil type, and to remove any circling or girdling roots. Poor root structure is a serious problem with nursery stock. Having roots growing in two soil types is also a serious problem. But you can't plant this way in summer! and I don't know anyone who does it, it just goes against everything we were taught. I come close when I whack back the rootball with a shovel or ax.

Now I've dug extra big holes for bare roots with beautiful root structure. I once got a La Ville de Bruxelles with huge, long, straight roots that spread symmetrically for a diameter of 4 feet. I dug a hole big enough to get all those roots into! they were gorgeous! and man did that plant explode out of the ground its first spring. And it was very drought tolerant its first summer too. I attribute both to having all those roots spread symmetrically through the native soil. But, an unusual case. Most roots aren't in good enough shape to do that with.