"I had a little bit of success here," he said Wednesday. "I get to come back and work on some things I didn't do well here I worked on all the time I was off. I come back a little more focused and a lot more determined."
Butler was signed by the New Orleans Saints last week but released a few days later.
The 30-year-old Lewis is a 6-foot-1, 222-pound strong safety who played under Steve Spagnuolo when the Rams coach was an assistant in Philadelphia. He was picked for the Pro Bowl after the 2004 season.
"Mike's a good football player," Spagnuolo said. "I think it was in 2002 we drafted him as a rookie. Happy we got him. He played a lot of good football for us and he's continued to do that."
"As a football player, I'm a physical guy," Lewis said on Wednesday. "I like to get downhill and make plays and just do my job."
Lewis and Butler take the roster spots of defensive tackle Clifton Ryan and linebacker Curtis Johnson.
Ryan had been suffering from migraine headaches.
"It's just a matter of letting that thing clear," Spagnuolo said. "Obviously it was kind of extended. Cliff and I talked this morning, but it's the right thing for the player. It's about their health first."
Johnson was signed from the practice squad to the active roster last week, primarily to help with special teams, but was inactive against Tampa Bay.