Pretty smart that they have 3 good running backs isn't it. I bet if it were regular season and he was needed he would be playing. It's perfectly okay to be conservative with any kind of injury given it is preseason.
Back spasms is not an injury it's a condition that players play with.. Lynch deals with these year in and year out. no big deal, no need to push him to the point of a real injury or setback..
None of these little injury (Avril, Carp, Miller) reports worry me until they start keeping them out of real games...