Roses do not like root disturbance at the best of times, and any damage at this stage is another shock that they can well do without.
Depending on how close it is to successful neighbours and the risk of root disturbance, grub the rootstock out.
The seedlings need sterile soil to prevent damping off and resent root disturbance during transplanting.
They resent the inevitable root disturbance during transplanting, especially the poppies, with their tap roots, and often don't make it.
Also, root disturbance should be minimized.
Immature composts may cause settling and young root disturbance due to oxygen deprivation.
Division is also possible although great care must be taken as the plant hates root disturbance.
That way you avoid the hazards of pricking out seedlings and keep root disturbance to a minimum.
It requires acidic soil (pH of 4.5 to 6) and does not tolerate root disturbance.
The fungal and bacterial agents need to be present, and root disturbance is often fatal.