Do we even know if Rosenstein even saw them before he appointed the special counsel? Mueller's had these memos from the beginning. Comey met with him before he testified to Congress, and I read he turned them over to Mueller. So if Mueller has had these, he's known all along that there wasn't any obstruction or collusion.
Mueller works directly for Rosenstein. The leaked Comey memos to the NYT via his law professor friend is what set off the appointment of a special counsel, which was Comey’s stated objective as he testified before Congress.
I have no doubt that Rosenstein read the memos before he appointed Mueller.