No. Unless you have objective evidence such as direct visual observation that the back pain is interfering with the employee’s work, you should leave it alone. If at some point in the future it becomes apparent that the employee is having issues while working (for example, unable to lift as usual, holding their back, groaning, etc.), then you should definitely speak with them to understand if there may be some restrictions that would affect their ability to continue doing the essential functions of their job. If so, you should discuss whether there is a reasonable accommodation you can offer to make that possible. You may be able to request documentation from the employee’s health care provider concerning applicable restrictions and duration to help with this process and determination.
So, unless a disability becomes apparent through observation, or is reported to you by the employee themselves, their family member, or a doctor, you should not assume one exists or that it would affect their ability to perform their job. Doing so can lead to discrimination claims as well as poor morale and hurt feelings.