-___- Guys, it's been speculated a long time ago Minato could enter Sage mode.
The reason this wasn't mention was due to the fact Naruto hadn't learned who his father was. However Minato, was a pupil of Jiraya's long before Naruto. Minato was able to master Sage Mode, as well. Although he may have not used it during his fight with Obito, during the Nine Tails attack. The fact of the matter is you have to be standing completely still in order to enter Sage mode, this couldn't possibly have happened when he fought Obito due to Minato having to keep the nine-tails sealed, protect Naruto and Kushina, while at the same time prevent Obito from taking contol of the nine-tails. Unfortunatley for him he failed, so sealing the Yin half of Kuruma within him. He sealed himself within the Shingami, where I'm assuming he obtained control of the Yin half of Kuruma's chakra. considering his soul was sealed for 16+ years.
So, I say it's plausible for Minato to have enter Sage Mode as he did, and when he did considering he didn't have Yin Kuruma's chakra to rely on anymore.