49ers place Drake Jackson on injured reserve, will be out at least four games

Defensive end Drake Jackson, who opened the season with a three-sack game against Pittsburgh, was placed on injured reserve Saturday and will miss at least the 49ers’ next four games because of a knee injury.

Jackson, last year’s top draft pick in the second round, had seen his playing time dip significantly in his last four games. He had opened the season on a rotation basis with Clelin Ferrell opposite Nick Bosa, and Jackson figured to be bumped from their game-day lineup following the Oct. 31 trade for defensive end Cameron Young.

Jackson played just 10 snaps in the 49ers’ 31-17 loss to Cincinnati before the bye, and a knee injury landed him on this past week’s injury report. He was limited Wednesday and Thursday before not practicing Friday and going on Injured Reserve some 24 hours before the 49ers (5-3) visit the Jaguars (6-2).

After four games, the 49ers could choose to activate Jackson to the active roster or leave him there for the remainder of the season, depending on the severity of his injury and their need for a backup on the defensive line.

After fading during his rookie season a year ago, Jackson, along with Javon Kinlaw, drew raves for their offseason and training camp work by the coaching staff. Jackson, in particular, was praised by the coaching staff for remaking his body after being a healthy scratch in four of the 49ers’ last five games (including the playoffs) a year ago.

Jackson had three sacks in a season-opening 30-7 win over Pittsburgh and played 28, 40, 30 and 41 snaps in the first four games. In the last four, Jackson’s snap count dropped to 28, 30, 15 and 10.

Cornerback Shemar John-Charles was activated from the practice squad to assume Jackson’s roster spot. The two standard practice squad promotions for the Jacksonville game are wide receiver Willie Snead IV and linebacker Curtis Robinson.