The Steelers are in must-win mode win at 0-2. The 49ers are in a letdown spot returning home after opening 2-0 with a pair of road victories. Those wins came against the Buccaneers and Bengals, though.
Now the 49ers face their toughest opponent. The line is inflated because Ben Roethlisberger is out. He's replaced by Mason Rudolph. The Steelers have had bad backup quarterbacks before. Rudolph is a step above their previous second-string QB's. He was a star at Oklahoma State and looked good during preseason. The Steelers still have a good defense, an elite offensive line and solid weapons for Rudolph to succeed. 
San Francisco is much improved. But the 49ers still are not better than the Steelers even with the change from Roethlisberger to Rudolph. Their offense is a work-in-progress and they just lost offensive left tackle Joe Staley, who suffered a broken leng. The 49ers have holes on defense especially in the secondary. 
The Steelrs upgraded their secondary and showed to their fans they haven't given up on the season by trading a first-round pick for versatile defensive back Minkah Fitzpatrick.