Roethlisberger finished 36 for 51 for 383 yards, with one touchdown and one interception.
The Steelers are still looking for their first turnover of the season, as they failed to recover Cassel's fumble on the Vikings 38 in the second quarter, with the ball bouncing forward before being recovered by Jerome Simpson for a 10-yard gain.
"We came up short, but we played better than we have in recent weeks," Steelers head coach Mike Tomlin said. "We gave up explosive plays, you can't do that. ... We're going to focus on getting better, and that's what's going to change the outcome of football games."