Coach Mike Tomlin called the injury "minor" after the game.
Pittsburgh was driving early in the second quarter, but Ward could not hold onto a pass near the first-down marker on third-and-7 due to a helmet-to-helmet hit from Lewis. Team trainers attended to Ward before helping him walk off the field.
Ward sat on the sideline without his helmet and never returned as the Steelers lost 23-20 to division rival Baltimore.
There was no penalty on the play, but Ward appeared to fit the "defenseless receiver" scenario instituted by the NFL last season.
Ward missed last week's 25-17 win over New England due to an ankle injury.
Information from The Associated Press contributed to this report.