Lewis did not give a timetable for Green's return, but said he should be back by the end of the season.
The only positive news regarding Green's injury, Lewis said, is that the hamstring isn't torn.
"If there was any positive news, it would be that," he said. "I think he's moving today maybe better than even expected, so that's good."
Green was injured Sunday against the Buffalo Bills, when he hit the ground trying to make a catch on the second play of the game.
Lewis also confirmed running back Giovani Bernard has a torn left ACL. Lewis said Bernard is currently exploring his options, but will likely have surgery.
Bernard was in the Bills game until the last minute, and when he got hurt is unclear. Teammate Rex Burkhead said he believes Bernard got hurt on a play on 1st-and-10 with 1:20 left, when guard Clint Boling fell on top of him after he made a catch.
This is the second time in his career that Bernard has torn an ACL. He also tore his right ACL in 2010 while at North Carolina, but came back to rush for 2,481 yards and 31 touchdowns in his two seasons at UNC.
Bernard and Green lead the Bengals in receptions.
"They've accounted for a lot of what we've done this year," Lewis said. "But we've got other guys that have to step up and get going."
Bengals cornerback Dre Kirkpatrick, safety Shawn Williams and long snapper Clark Harris were also injured. Kirkpatrick, who injured his knee, said he was all right, but still experiencing pain and that it's up in the air if he plays on Sunday.
"They're kind of all day-to-day," Lewis said of the other injured players. "We'll see how they come through the week. ... But it was obviously the most significant day we've had as far as injuries go in one game."
Green was around the facility on Monday afternoon and briefly stepped out of the training room into the locker room, but didn't comment on his status. Bernard was not present during the open locker room period.