Do you think that Kakashi has something like CS or SageMode , ofcourse it can be used only by Kakashi. I think he has. Why ? Kakashi is son of Hatake Sakumo- shinobi which was strong like Sannin's so he could have something like that and prepare Kakashi to using it. (Not by trening but like Itachi told Sasuke about MS) Another proof is that Kakashi mastered MS with his bad stamina ? How ? If he is using MS he is going to hospital for a weak. Kakashi didnt have time for training in time skip he had to doing missions so in my opinion he had to train in his SSJ mode.