Lewis, 26, started five games for the Bills last season, finishing with a 2-3 record. His performance slipped this preseason, when he went 17-for-33 passing for 168 yards and two interceptions.
On Monday, Bills coach Doug Marrone voiced displeasure about the play of backup quarterbacks Jeff Tuel (73.9 QB rating this preseason) and Lewis (41.0 QB rating).
"I think it's what you see. They either have to play better or we have to look to see if there's someone that can better that position, like we would any other position," Marrone said. "If we feel at the end of the day that it's not good enough to win with -- which we haven't seen that yet -- there's only two ways to go.
"Either you put a lot of work into those guys and getting them better until you get to that position where you feel comfortable with it or you have to upgrade that position some other way."
Throughout the offseason, general manager Doug Whaley expressed confidence in Lewis and declined to bring in another backup quarterback to compete with Lewis or Tuel.
"Our philosophy for Thad is, you need your second [quarterback] to keep you .500. He got us to 2-3, so we're happy with that," Whaley told ESPN in May. "If he could have started another game and got us 3-3, 4-3, great. We're excited. We didn't think it was worth bringing in a veteran quarterback."
The younger brother of Arizona Cardinals quarterback Carson Palmer last appeared in a regular-season game in 2010 for the Bengals. In four career appearances, he has completed 10 of 15 passes for 59 yards.
Part of the Bills' thinking in signing Palmer could be to add a veteran tutor for Manuel. The team hired Todd Downing, who coached Manuel at the 2013 Senior Bowl, as its quarterbacks coach this offseason.