It was previously reported that Sagemiller would be replacing Paula Patton in the role of Assistant District Attorney.
"Right before Paula started her first episode... she was offered a test for Mission: Impossible 4," Baer told TV Guide. "We had a contract, but I don't want to stand in someone's career path if they're going to be in a huge feature film with Tom Cruise. So when she got that role, we [decided to] recast."
The producer added that Sagemiller's audition had featured on "a huge disc of performances" that the production team had compiled prior to casting Patton.
"We went back to someone we had liked early and that was Melissa Sagemiller," he explained.
The actress will now play ADA Gillian Hardwicke in the NBC crime drama.
"[The character is] a Brooklyn ADA who wanted to come to SVU because she greatly admires Benson and Stabler," revealed Bauer. "[She has] got an amazing track record in solving cases."
The new season of Law & Order: Special Victims Unit begins on September 22. Sagemiller's first appearance will air on October 20.