Second Opinion in California
According to this article, the answer would seem to depend upon whether or not your employer has created a "Medical Provider Network" (MPN).
Quote:
Quoting Second and Third Opinions:
If the covered employee disputes either the diagnosis or the treatment prescribed by the treating physician, the employee may obtain a second and third opinion from physicians within the MPN. During this process, the employee is required to continue his/her treatment with the treating physician or a physician of his or her choice pursuant to section 9767.6.
It appears that you are entitled to get a second opinion, but from a doctor selected from a pre-approved pool. Your own doctor can of course continue to treat you, and the comp doctor will probably be willing to review that doctor's assessment and the MRI results. A local workers' compensation lawyer should be able to analyze the comp plan you are under and be more specific about your rights.