The lack of consistent play in the trenches forced the secondary to remain in coverage against receivers longer and allowed running backs to reach the second level of the defense.
Much of the 49ers' defensive line struggles stemmed from the loss of defensive end Nick Bosa, who played in just three games before suffering a season-ending ACL injury.
Bosa is expected to be ready for the start of the 2026 season, which should provide a significant boost to the unit, assuming he can return to his previous form.
PFF's Gordon McGuinness ranked every NFL defensive line heading into the 2026 season and placed the 49ers' unit at No. 20. He believes the addition of defensive tackle Osa Odighizuwa should help improve the pass rush. The veteran has earned at least a 70.2 pass-rush grade in each of the last four seasons, including at least 78.5 in two of the last three.
Still, questions remain, even with Bosa returning.
"A fully healthy Nick Bosa would push this group much closer to the top 10 than the bottom 10," McGuinness wrote, "but he is coming off a season-ending injury and also missed multiple games in 2024."
Much of the 49ers' defensive line improvement will depend on the continued development of several young players, including recent draft selections.
"San Francisco would also benefit significantly from an immediate contribution from rookie Gracen Halton or a second-year leap from Alfred Collins," McGuinness added.
Collins and fellow second-year defensive tackle C.J. West showed flashes of potential in 2025 but ultimately finished as the 49ers' two lowest-graded players that season, each earning a disappointing 35.0 defensive grade from PFF.
The 49ers' No. 20 ranking reflects a defensive line with plenty of uncertainty but also significant upside. A healthy Bosa, the addition of Odighizuwa, and improvement from younger players such as Collins and West could dramatically change the outlook.
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