Mettenberger said the swap was 'just a little joke played on me by the real No. 7."
"I think this is the last day I will wear that number," Mettenberger said.
Mettenberger, cut by the Chargers in the preseason, isn't worried about getting his old number back on a full-time basis.
"I don’t think I have enough saved away so that I could even offer [Roethlisberger],' Mettenberger joked.
Mettenberger has been released by two teams since his 263-yard, two touchdown performance as a fill-in starter against the Steelers in 2014.
Mettenberger hopes to reset his career in Pittsburgh, which is near his father's hometown of Steubenville, Ohio.
Mettenberger is a tall pocket passer who likes to throw the deep ball.
"Obviously, our GM [Kevin Colbert] saw something he liked," Mettenberger said. "All I can do now is work hard and show he made the right choice."