Marrone acknowledged the Bills are in a tough spot at quarterback.
"It's difficult when you get down to your third or fourth quarterback," Marrone said.
And he defended the decision to open the season without a veteran quarterback by noting the Bills had to commit roster spots to address injuries at defensive back and with rookie kicker Dustin Hopkins sidelined by a groin injury. In promoting Lewis, the Bills placed Hopkins on season-ending injured reserve.
"Now, if you would've said to me, 'Hey, you're going to have two injuries (at quarterback), should you have done something different?' Absolutely," Marrone said. "But that really wasn't the plan going in."
Lewis played college at Duke and split most of the past three seasons on the St. Louis Rams' and Browns' practice squads. He has played one game, a start in Cleveland's season finale against Pittsburgh last year, when he went 22 of 32 for 204 yards passing with a touchdown and interception.
Calling his excitement level as being "through the roof," Lewis is attempting to keep his emotions in check.
"A lot of people are like, 'Why aren't you happy and jumping for joy?'" Lewis said. "But I know it's a business. And I know I have to get ready to help this team go out and win."
Lewis appeared in the Bills' preseason finale, going 7 of 16 for 132 yards and a touchdown in a 35-13 loss to Detroit. The Bills thought enough of his performance to sign him to the practice squad and cut fellow newcomer Matt Leinart.
"I just have to go out and put my best foot forward," Lewis said. "Don't do too much because you've got playmakers around you. But do enough to put us in a good position to get this 'W.'"