Braelon Allen ran for over 1,200 yards last year and almost 500 so far this year, averaging over 6 YPC both seasons. I can't answer to the 2 yards on 8 carries, but I can also say that Derrick Henry isn't rushing for over 100 yards every game, but still manages 1,500+ yards in a season and a rushing title.
You're not talking about one game though, you're speaking to the past few have gone down hill, but it's more on Mertz than anyone else. I don't see them have a 1,200 yard back as an offensive line problem. I think 60% completion rate and 10/11 TD/Int ratio is the issue. With a stud back, it should EASILY help your passing game open up regardless of talent.