Wasn’t Rosenstein’s call. Barr told Trump if Trump wants him Barr gets to choose his own staff and Trump agreed.
As Barr replied to Kamela Harris, Rosey was cleared of conflicts (being a witness to Comey’s firing) back when he assumed supervision over Mueller and became Acting Attorney General, long before Barr arrived on the scene.
This conflict did not arise from Barr’s appointment and had nothing to do with Barr.