Why was Obito's LEFT arm made out of Zetsu goo when he fought Minato?

[Altaneen]

There is a chance that it was Zetsu who battled Minato and not Obito.

No, no there is not.

 

[OWanime]

my guess is that zetsu after absorbing obito's chakra, left to fight minato; obito looked pretty determined to leave so they could just be using him as a prisoner ad using zetsu to access his powers xd