In Chapter 661, Hashirama said "These rods are forced through our chakra points, which means that the chances of us moulding chakra are probably nil". He said it in the context of "even if the rods are removed". Since those under Edo Tensei can recover on their own after some time, what did he mean by this?
While people do regenerate under Edo Tensei, the process of regeneration can be slow and tedious. Madara has reached new heights and can attack at any given moment. Hashirama knows his chakra circulation is obstructed and it could take a while for him to heal fully. Thus, he is giving Sasuke this precaution along with a jutsu.
Refer to Minato's statement below. Also recall how Minato lost his arm and still hasn't regenerated yet. Hashirama knows that if Madara acts quickly, he won't be able to stop Madara.