But if the sphere is already in the creature's square, then isn't the condition continuously fulfiled until the creature moves out of that space?
If your answer is 'no' as I think it will be, then does that mean the sphere needs to be moved out of the square, then back in again to have a chance to do damage? Or does the spehere need to simply move around within the square to attack again? If the latter, then how long would the spellcaster need to concentrate on that? Just as much as needed to move the sphere as a full move action? Seems unlikely.
I agree with Stalker0, semantics can be argued endlessly, but I think common sense needs to overrule missing text or ambiguity, be it 'right' or 'wrong' or 'as written' or not. I've heard you shouldn't mix common sense with D&D too much, but there are instances such as this where it seems rather silly to do otherwise.