James Harrison says it ain't just Big Ben and the Steelers' D causing problems in Pittsburgh ... 'cause the legendary LB tells TMZ Sports it's time start pointing fingers at Mike Tomlin as well.
"I think now, a lot of it has to be put on to what the game plan is -- the coaching."
Tomlin's been Teflon in Pitt -- the dude does have a Super Bowl ring -- but a lot of people ... including Terry Bradshaw himself .