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.
Note that this was posted on March 4th of 2017, when it's almost certain that Trump learned of this surveillance when Admiral Mike Rogers paid that remarkable visit to Trump Tower on November 18th of 2016.
Take everything Donald Trump says with a huge grain of salt, dude ... especially when he says it in an interview with a media outlet that he has always referred to as "the failing New York Times!" LOL.