A sixth-round selection in the 2014 draft, Mettenberger finished 0-10 as a starter in Tennessee and did not play in the past two preseason games for San Diego.
In his only preseason action, Mettenberger completed 2 of 8 passes for 22 yards, with no touchdowns and one interception against the Titans.
While Ben Roethlisberger is entrenched as the Steelers' top quarterback and Landry Jones as his backup, the team has had issues with its third-string position. Veteran Bruce Gradkowski and project Dustin Vaughn were both in camp with the team, but each has been placed on injured reserve. Gradkowski, according to a source, suffered a hamstring tear during camp, while Vaughn has a thumb injury.
Information from ESPN's Eric D. Williams and Jeremy Fowler contributed to this report.