Cohn's take:
Grade: C.
Offensive line is one of the 49ers' biggest weaknesses.
The Niners' offensive line gave up 52 sacks last season—second most in the league. Only the Jacksonville Jaguars gave up more sacks than the 49ers.
As recently as 2012, the 49ers had the best offensive line in the NFL. What happened?
Joe Staley and Anthony Davis still are excellent tackles, but Staley is on the downside of his career, and Davis might be injury prone. Their backup, Jonathan Martin, is just bad.
Starting right guard Alex Boone also seems to be on the downside of his career. He was mediocre last season after returning from a holdout. And Pro Bowl left guard Mike Iupati signed a five-year deal with the Arizona Cardinals, according to Adam Schefter.
The 49ers needed to sign an offensive lineman. They signed Erik Pears for two years, $.7 million according to Niners Nation. Pears isn't great or even particularly good, but he can play guard or tackle, and he's better than Jonathan Martin. Now, the Niners can cut Martin, which is a bonus.
http://bleacherreport.com/articles/2399207-grading-the-san-francisco-49ers-free-agency-moves-so-far/page/7