No, they do not in Illinois unless your job is protected by a collective bargaining agreement or some other contractual provision. While your employer does not have an obligation to keep your job for you if you are disabled on account of a workplace injury they still have an obligation to pay you your workers’ compensation benefits when you’re off work.
Your employer cannot fire you strictly for filing for workers’ compensation, but if they need your position filled and you’re unable to work, they can lay you off and fill that position. It does not, however, absolve them from having to pay your workers’ compensation benefits while you’re unable to work.
If you find yourself in a situation where you’ve been injured at work and are unable to return to your regular duties, you should speak with an experienced workers’ compensation attorney about your right and options.