Only the Bills medical staff knows exactly what's wrong with Bell's back. (By the way congrats to Tim Graham who was the first to report Bell might replace Walker 1-2 weeks ago). I'm willing to guarantee Bell had a MRI and with Walker's release felt the injury not severe. It could have been a strain, sprain, herniated disc, or whatever. I suspect it was a strain and is expected to get better with NSAID's(like ibuprofen), rest, and physical therapy. There's always a chance it can be re-aggravated especially shortly after healing.