That assumption is nonsense, for many reasons.
If Mueller's investigation into Trump campaign connections into Russia, which is what his authorization loetter has him looking into, why on the world would he add another investigation so as to disqualify himself from the whole thing? Why would Rosenstein do that?
The Trump obstruction claim can stand separate - it is a view into Trump's conduct - from the Trump campaign collusion claim. Mueller is tasked with looking into the collusion claim(s).
Rosenstein has already made two astonishing admissions to the press, which aren’t being closely covered of course. First, despite the language of the appointment itself, Rosenstein has admitted that he will leave any decisions on whether to expand the investigation to Mueller. He went on to say that if Mueller decides that Rosenstein should recuse himself, Rosenstein would, but is so far appearing to leave that up to Mueller.
It’s as if Rosenstein is star struck, or something more sinister such as being part of a plot, but has basically already recused himself from the process. These admissions would make a very good case for firing Rosenstein for dereliction of duty, and put someone in there who could hopefully keep Mueller on a short leash, if not shut him down completely.