I don't trust those quotes nor how the conversation was intended....nobody is going to be guaranteed a starting spot going into that season unless you were the clear out and out better option, which was never the case with Henderson. To that, it's extremely hard to not start your highest earning player in that position, as long as they were not a total donkey like Sanchez and Lukaku became.
And that attitude of proving yourself to the new manager is the exact opposite of his quotes. Yeah, let the manager see what I have got because I can prove it to him. And it's not only just one a time thing, the guy is contracted to United longer than DDG is at this point.