Allen is slated to start against the Dolphins on Sunday, Matt Barrows of The Athletic reports.
The 26-year-old hasn't appeared in an NFL game since suiting up for the Steelers in 2018, but San Francisco finds itself in dire need of secondary help heading into a matchup against Miami's 20th-ranked pass offense. The 49ers are dealing with a cluster of injuries to the back end of the defense, with K'Waun Williams (knee-ACL), Richard Sherman (calf), Emmanuel Moseley (concussion) and Dontae Johnson (groin) all unavailable for Week 5. Allen was on San Francisco's practice squad as of Saturday morning, but with a seemingly strong week of practice, he has been activated and moved ahead of Ken Webster on the CB depth chart.