Mullens will start Sunday's game against the Giants, Matt Maiocco of NBC Sports Bay Area reports.
The 25-year-old has been taking first-team reps at practice this week with Jimmy Garoppolo (ankle) sidelined, and he'll receive his first start since 2018 now that Garoppolo has been ruled out. Mullens completed eight of 11 passes for 71 yards with one interceptions when filling in last week, but he should be more prepared with a full week of practice. He averaged 284.6 passing yards, 1.6 touchdowns and 1.3 interceptions over eight starts in 2018.