McClain reports that they don't expect for Cody to miss the first regular season game.
The most likely is that he has suffered an acute lumbar disk herniation or a lumbar muscle strain. The epidural (which usually includes a steroid and local anesthetic) is to decrease the inflammation and swelling around the involved nerve roots or the muscles, and therefore decrease the pain. It does NOT treat a bulging disk. It does not cause a bulging disk to decrease in size. It is meant to allow rehab and, in the case of a disk problem, the hope to avoid surgery. Decreasing the pain does not ensure that he will do well with rehab, nor does it ensure that he will avoid surgery. We don't know what the MRI showed. Anyone who has ever suffered a low back disk herniation or to a lesser degree in the case of a lumbar muscle strain, knows how unpredictable its rehab can be, and how easily it can be re-injured. Unless, this is a lumbar muscle strain or a very minor disk, I cannot see how the Texans can predict date of return for the first regular season game. If this injury by any chance involves a lumbar compression fracture, all bets are off.
Here is a recent piece (presented at the American Orthopaedic Society for Sports Medicine February 2012) that will give you something to hang your hat on, as relates to lumbar disk herniation
Epidural steroid injection returns most professional football players to the field
EDIT: lumbar muscle strains can also be accompanied by lumbar sprain (vertebral ligament damage)