The team fears Smith has a torn triceps, and he will undergo further tests Monday. However, the Steelers are operating under the assumption that Smith will be out for the season, according to the source.
Smith played in the Steelers' first five games in 2009 before sustaining a torn rotator cuff against the Detroit Lions. The Steelers placed him on injured reserve following that game, ending his season.
Steelers linebacker LaMarr Woodley (hamstring) and tackle Flozell Adams (ankle), who also were injured in the 23-22 victory over the Dolphins, aren't out long-term. Woodley said he'll "definitely" be back next week. Adams also could return by then.
Coach Mike Tomlin said he didn't know the extent of Woodley's injury, but didn't sound optimistic about Smith.
"Not good, but such is life in the National Football League," Tomlin said. "We will adapt and hopefully overcome it."
The Associated Press contributed to this report.