Trainer Lonnie Soloff confirmed Sunday that an MRI showed no structural damage but some inflammation.
"Scott has a moderate left forearm strain," Soloff said. "We hope for an expected return of two to four weeks. This is highly encouraging news."
The Indians were concerned that the injury was more serious when they placed the 25-year-old on the 15-day disabled list Saturday. Lewis said then that he had felt some discomfort for two or three weeks.
"I've pitched through some soreness and it usually fades away," said Lewis, who had Tommy John surgery as a sophomore at Ohio State in 2003. "I thought this would be the same, but on my last pitch, it really tightened up on me."
Lewis threw 94 pitches over 4 1/3 innings Friday against the Toronto Blue Jays.