Joining Lewis on IR was starting safety Brodney Pool, who sustained at least his fourth known concussion during Sunday's loss to the Cincinnati Bengals.
"After consulting with our medical team, we felt that this was in the best interests of both Jamal and Brodney at this time," Browns coach Eric Mangini said in a statement released by the team. "As I've said before, organizationally, players' health and safety are paramount in any decision we make with regards to putting them back on the field."
Lewis' agent, Mitch Frankel, didn't immediately return phone messages seeking comment.
A punishing runner who has battered would-be tacklers since he entered the NFL in 2000, Lewis has been bothered by assorted leg injuries and said last month that he intended to retire after this season.
Lewis has rushed for 10,607 career yards, ranking him 21st on the NFL's all-time list and just 36 yards behind Ricky Watters (10,643) for 20th place. Lewis was the league's Offensive Player of the Year in 2003, when he rushed for 2,066 yards with the Baltimore Ravens.
Unfortunately, Lewis' final season has been his worst. He has rushed for 500 yards on 143 carries -- a 3.5 average -- and hasn't scored a touchdown. He didn't start last Sunday at Cincinnati, but he came in for the second offensive play and finished with a team-high 40 yards on 11 carries.
Lewis' final carry was a 1-yard run with 14:02 left in the game.
Earlier this season, Lewis, an offensive co-captain, criticized Mangini for working Cleveland's players too hard in practice. Lewis also questioned the team's lack of an offensive identity. He backtracked on his comments a few days later, blaming reporters for "blowing it out of proportion."
Lewis seemed content with the idea of retirement and said he was looking forward to life away from football. He owns several lucrative businesses, including a trucking company based in Atlanta.
While Lewis' career is in question, Pool also is facing a difficult decision on his future. A five-year veteran, Pool suffered at least his fourth known concussion during the third quarter of Sunday's game.
Mangini said in a news conference earlier Wednesday that no decisions have been made about Pool's future beyond 2009.
"We haven't talked about the long term," the coach said. "That's a discussion for a later time period. You never want to have a conversation about injuries with anybody, but unfortunately it's something that is a reality, and it's never a fun conversation, but they are always important."
Pool has started 10 games this season and 49 since the Browns selected him in the second round of the 2005 draft out of Oklahoma. In last Sunday's 16-7 loss to the Bengals, Pool was injured early in the second half. He walked off the field, was escorted to the locker room and didn't return to the sideline.
Pool's situation has many of his teammates concerned about his health -- and their own. With concussions a hot topic in the league, players are learning more about the dangers of head injuries and their lasting effects.
Browns linebacker David Bowens has seen a big change in the way that teams are treating concussed players.
"Early in my career, it was like, 'OK, he's got a concussion, can he count to three?' Now it's a big deal," Bowens said. "You see how the older players, how it's affected their lives post-career and it is a serious issue. I think the awareness level has definitely heightened because of it, and the teams are taking better precautions. Safety issues regarding helmets, that's gone up. There's a lot of steps being taken."
Browns wide receiver Mike Furrey said he had a Tuesday conversation with former St. Louis Rams teammate Kurt Warner, who missed the Arizona Cardinals' game against the Tennessee Titans last weekend with post-concussion symptoms.
"I don't think anybody really has an understanding yet of the symptoms and why things are happening the way they're happening," Furrey said. "There's no answers."
Furrey said it's not uncommon for a player to lie to a team doctor to keep playing.
"When you're talking about a little headache throughout the week, you obviously feel like you're still going to be able to play on Sunday," he said. "But nobody knows the extremities of those headaches or the difference of a light one or a heavy one or not having any other symptoms, being nauseated and sick throughout the week.
"As a competitor, you want to keep playing, so obviously you probably would stretch the story just a little bit to get back on the field, and it's just the nature of any athlete that wants to play on Sundays."
Notes: The Browns were awarded DB Ramzee Robinson off waivers from the Philadelphia Eagles. He also has played with the Detroit Lions. ... The Browns elevated DL Brian Schaefering to the active roster and signed RB Jed Collins and DB Michael Grant to the practice squad.
The Associated Press contributed to this report.