I rule out Harris because NE's usage of him this year has been rather inconsistent. Even if Houston should be an easier test, NE might just as easily decide to let his be a game where Mac Jones gets to boost his confidence by giving him big passing numbers. Harris has the highest ceiling of the three, 100 yds and a TD or maybe 2. But you just as easily could get 10 rushes for 29 yards.
That leaves the backup RB ascending to a new role with a rookie QB vs. Pittsburgh's #1 receiver. Roethlisberger is cooked, but I think you'll get a good floor and the possibility of a TD to boot. Johnson would be my choice.
In non-ppr Dam Williams (w/Montgomery out). Dionte is great in ppr formats cuz of the volume of targets but I don't know that I would trust him otherwise. Especially, going up against a DEN D and w/Big Ben's wonk shoulder. PITT may not have to pass a lot in this contest w/DEN having to start Drew Lock. May be a very low scoring contest.