He was unaware that the prime realityMichael Burnham had infiltrated the vessel and taken her counterpart's place. Burnham named the mirror Saru with the name of his primary universe counterpart and claimed it was in tribute to a respected friend. Later, the mirror Saru saved Burnham's life when Ash Tyler's programming was undone and Tyler attempted to kill Burnham.
When consulting with the primary Saru, Burnham decided not to tell him about his counterpart and his place aboard the mirror vessel. (DSCepisode: "The Wolf Inside")