To make room for Wallace on the roster, the Packers released quarterback B.J. Coleman.
Wallace was with the team for its practice Monday. The San Francisco 49ers released him last week after he reportedly informed coach Jim Harbaugh that he planned to retire.
Wallace said Monday that San Francisco "wasn't the right fit" and that he never said he was retiring.
"No, no. It's just miscommunication between me and Harbaugh. We talked about it. I called him to communicate that to him. It was just pretty much miscommunication between me and him," he said.
Harbaugh, however, told ESPN on Monday that he "relayed the information as it was presented to me," when asked whether Wallace told him he was retiring.
Wallace was asked about his confidence level if he had to play for the Packers soon.
"My confidence level, I just have to lean on my experience and I'm pretty sure if something was to happen, God forbid, that we would try to lean on some of the stuff that I learned in the past, just to try to get through it. But we're not going to talk about that. We want the man to stay upright and continue to keep doing his thing," he said.
The Packers wanted a veteran capable of helping them win if Rodgers is injured.
"Seneca Wallace has been in the league 11 years. He's been a backup quarterback primarily his whole career. He clearly understands the role, has a lot of experience in an out of systems. Just talking football with him last night, I felt confident that he'll be ready to play Sunday. That was part of our conversation," coach Mike McCarthy said.
The Packers released Vince Young among their final roster cuts. In discussing the roster moves on Sunday, Packers general manager Ted Thompson placed the blame on himself for not acting sooner to bring in the former first-round draft pick.
"Quite frankly, it probably wasn't fair to Vince," Thompson said. "We threw a lot on his plate, and the fault is probably mine. I probably should have had him in here earlier."
The Packers open at San Francisco and now have two quarterbacks on their roster who spent training camp with the 49ers -- Wallace and Scott Tolzien, who was signed to the Packers' practice squad. Both could assist the Packers in preparations throughout the week.
ESPN.com Packers reporter Rob Demovsky contributed to this report.