Oh right. Yet he was wasting his time to somehow cause a deadly blow to juubi jin? Did he not see that even PS sword had little to no effect U_UMinato knew that obito was a Space/time Ninjutsu user, He knew that Nothing will work on him except FTG, thus he never wasted his time going into sage mode.
At this point it's all speculation on why he didn't use it, Point is, it was obvious he was a sage.
Just fanservice U_U