"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."
Palmer, 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 their quarterbacks coach this offseason.
Earlier Monday, the Bills waived veteran Dennis Dixon, who was listed as fourth on their quarterback depth chart.