Three points.
1. Opponents found that eight-man fronts had some success in stopping Peterson and Taylor late on last season. You can guarantee most defensive coordinators will be focussing on stopping the run against the Vikings, and Minnesota did add Berrian to their WR corps, whilst Rice might progress in his 2nd year.
2. He will get quite a lot of rushing yardage and possibly a handful of rushing TDs too. In fact, if anything I think most of the FBG projections underplay this possibility. He rushed for 176 yards over the last six games last year when he resumed being the starter. That would translate to nearly 450 rushing yards over the season. Throw in the likely three or four rush TDs and that's roughly the equivalent of an extra 1500 yards passing. That's why he ranked the number 5 QB in my league over that last six-week period. Remember, Kordell Stewart wasn't much of an NFL passer, but for a year or two he was a really good fantasy passer, and Jackson is someone you might be able to get late, because the general view of him is that he can't play.
3. As the poster above states - read the FBG magazine.