Shortly before 8 a.m., coach Doug Marrone pulled Lewis aside to inform him to get ready to face the Cincinnati Bengals on Sunday.
"He just told me, 'You're starting,'" Lewis told The Associated Press. "He gave me the nod, so I have to go out and do my best with it."
A day after working out free agents Pat White and Dennis Dixon, and losing out to Minnesota in a bid to sign recently cut Tampa Bay starter Josh Freeman, the Bills (2-3) stayed in-house in finding an interim replacement for injured rookie starter EJ Manuel.
"We thought it was best for us to move on and play with Thaddeus," Marrone said. "With Thad, he knows the whole system inside out, so at least we can give him the things that we feel he's strong with, which gives us a better chance."
The scramble to find a starter began after Manuel sprained his right knee in a 37-24 loss to Cleveland on Thursday. The injury will not require surgery, but Manuel is expected to miss a few weeks, and potentially longer.
The Bills had only one other quarterback on their roster, undrafted rookie Jeff Tuel, who struggled in replacing Manuel on Thursday. Tuel completed 8 of 20 passes for 80 yards and an interception that was returned for a touchdown.
Lewis' promotion came as a surprise after Marrone indicated last week that the team intended to look outside the organization for a replacement. Those plans changed Sunday night, once Freeman announced he was signing with Minnesota.
Lewis has spent the past six weeks with the Bills. He was acquired in a trade with Detroit in late August, after Manuel was sidelined with a left knee injury and veteran Kevin Kolb sustained a concussion.
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.'"
Marrone said it was still too early to establish a timetable on Manuel's recovery.
He was able to provide more encouraging updates on the status of other injured players.
Starting safety Jairus Byrd is expected to play Sunday after missing the first five games with plantar fasciitis in both feet. Starting cornerback Stephon Gilmore (broken left hand) and backup Ron Brooks (broken right foot) are scheduled to practice this week.
Marrone listed receiver Stevie Johnson as day to day after the starter hurt his back against Cleveland.
NOTES: Marrone said the starting left guard is up for grabs now that Doug Legursky has returned from a left knee injury. Legursky will compete with Colin Brown. ... The Bills claimed OG Antoine McClain of waivers, two days after he was released by Oakland, and released backup OG Sam Young. ... Punter Brian Moorman, who re-signed with Buffalo on Sunday, has his old No. 8 back. Tuel gave up the number and will no wear No. 7.
AP NFL website: http://www.pro32.ap.org