All good points regarding the intricacies of the injury:
I am an athletic trainer, have been one for 10 years. As we previously stated, the achilles is part of the calf, so saying that an achilles injury is different than a calf injury is accurate, but they can also be one in the same - which is why my guess is that the celts refer to it as a calf injury.
Like any tendon or ligament, partial tears of the achilles are common...full fledged tears are some of the most painful obvious injuries that we see (reports from those injured state that it feels like a shot gun blast to the achilles, and in fact tends to be characterized by the lower calf muscle actually rolling up into the leg - similarly to what happens when you snap a window shade to roll it up). Partial tears, which is what was stated as what occured, can be played on. It certainly limits what someone can do, specifically in basketball where lift and mobility are two of the more important factors for functionality.
going on what Roy stated (and props to him for the accurate write up), this could certainly be an extension of the original injury. or it could be a secondary injury caused by the weakness from the initial injury. the reason i threw it out there is so that we as a community can come to grips with the fact that most likely shaq will not be back, and if he does come back, he will be severely limited.