Steinbach underwent back surgery on Tuesday to repair a "disk issue." Cleveland coach Pat Shurmur said the surgery went well, but that the team will place Steinbach on injured reserve, ending his season.
Steinbach left practice two weeks ago when his back stiffened up. He was receiving treatments before opting to have surgery.
Steinbach, 31, hasn't missed a regular-season snap in two years, playing between Pro Bowl tackle Joe Thomas and Pro Bowl center Alex Mack. Steinbach's quickness allows the Browns to use him on sweeps, and his blocks helped Peyton Hillis rush for nearly 1,200 yards and 11 touchdowns last season.
With Steinbach out, rookie Jason Pinkston is the most likely candidate to take over at guard, but Shurmur said the team will explore other possibilities.
The Browns were awarded former Baltimore Ravens offensive lineman Oniel Cousins off waivers late Monday, and Cousins worked at right tackle in practice Tuesday.
The 6-foot-4, 310-pound Cousins was projected to be the Ravens' starting right tackle heading into training camp. After one preseason game, he was moved to right guard, then demoted before being waived when Baltimore signed former Minnesota Vikings tackle Bryant McKinnie.
Steinbach has been with the Browns for four seasons. He signed with Cleveland in 2007 after spending four seasons in Cincinnati.
The Associated Press contributed to this report.