The employer needs to tread carefully, as the back injury could potentially be protected under the Americans With Disabilities Act (ADA), such that he can qualify for up to 12 weeks of unpaid leave. The workers compensation issue does not seem to be the primary problem, given that the worker returned to work without restriction, claims no residual injury, and with that absence being only a small part of the problem.
The employer should consult an employment injury about whether the ADA is likely to apply to the employee's medical condition before taking action.

