Steelers quarterback Dennis Dixon tore his lateral meniscus cartilage, a team source told ESPN NFL Insider Adam Schefter.
Dixon, who injured the same knee at Oregon in 2007, will get a second opinion this week, but if the initial diagnosis stands, Dixon will undergo arthroscopic knee surgery and miss at least three weeks.
By then, Ben Roethlisberger's four-game suspension will be over and he will return as the Steelers' starting quarterback.
The Steelers would not say how much time Dixon will miss.
With Dixon out, the Steelers re-signed quarterback Byron Leftwich two days after they released him to create a roster spot for a defensive lineman.
Leftwich, who may start Sunday's game at Tampa Bay, was let go Saturday in a procedural move so Steve McLendon could be activated for the Tennessee game. McLendon had a fumble recovery during the Steelers' 19-11 victory, but was released Monday so Leftwich could return.
Leftwich was expected to start throughout quarterback Roethlisberger's suspension, only to sprain his left knee in the Steelers' final preseason game Sept. 2. He hasn't played since.
Charlie Batch, No. 4 on the depth chart, finished up after Dixon was hurt Sunday during a 19-11 victory at Tennessee. He is currently the only healthy quarterback on the roster.
Information from ESPN NFL Insider Adam Schefter and The Associated Press contributed to this report.