The 49ers are moving forward without Elijah Mitchell for the next several weeks, reportedly adding veteran Marlon Mack to their running backs corps. Mack, formerly of the Colts and (briefly) the Texans, is expected to be signed to the 49ers’ practice squad, according to ESPN’s Adam Schefter. The 49ers have not yet announced the move. Mack was released from Houston’s practice squad last week after signing with the team in the offseason and missing the initial 53-man roster. A Colts fourth-round pick in 2017, Mack spent the first five seasons of his career with Indianapolis, often working in a r…