Trump also said he didn’t know who Rosenstein was ... even though it was TRUMP who rejected Rosenstein’s resignation as the U.S. Attorney for the Maryland district after SESSIONS ordered Rosenstein to submit his resignation as a matter of course.
Trump said that he didn't know that Rosenstein was from Baltimore. Trump didn't say that he didn't who Rosenstein was.
Trump likely kept Rosenstein on as US attorney because he was the only remaining Bush holdover.
Both of those things don't lessen the fact, that Trump has said that Rosenstein was a mistake.