A defense that proved to be among the worst in NFL history will be overseen by an unproven coach in 2017. The 49ers will hire former Jacksonville linebackers coach Robert Saleh to be their fourth defensive coordinator in the past four seasons, according to multiple reports. Saleh, 38, will be the NFL’s third-youngest defensive coordinator and the 49ers’ first rookie defensive coordinator since they hired Greg Manusky in 2007. The 49ers also allowed the sixth-most rushing yards (2,654) since 1990 and established an NFL record by allowing a 100-yard rusher in seven straight games. Saleh, who spent the past three seasons with the Jaguars, was a low-level Seattle defensive assistant from 2011 through ’13. Last week, Shanahan, who was the Falcons’ offensive coordinator, acknowledged that Atlanta’s deep postseason run put him behind. The 49ers announced the hiring of wide receivers coach Mike LaFleur, run-game specialist Mike McDaniel and offensive quality-control