Mullens will be signed from the 49ers' practice squad to serve as C.J. Beathard's backup following Jimmy Garoppolo's season-ending knee injury, Matt Maiocco of NBC Sports Bay Area reports.
The 49ers are also planning on bringing in a group of veteran free-agent quarterbacks to compete for the backup job Tuesday, so Mullens' stay on the 53-man roster may be short-lived. The 23-year-old completed 72.1 percent of his passes for 396 yards, one touchdown and three interceptions this preseason.