The losses for Utah are almost exclusively on the OL and DB (except Williams, who isn't as big of a loss, as I'll explain below). Utah actually returns most of their offensive production, and significantly more than BYU. Here are the numbers:

Passing yards:
Utah returns every single passing yard, 100% (2757 for Williams, 60 for Huntley) and passing TD (15)
BYU returns 241/2573 yards, or about 9.4%. Also returns 3/15 TDs, so 20%.
Now that's not entirely fair, since Tanner did start as a Freshman, but if Hill was really your best option, I wouldn't be as excited about Tanner as this board seems to be.

Rushing yards (from running backs, not counting QB runs):
Utah returns 201/411 carries (48.9%), 949/2356 yards (40.2%), and 8/18 TDs (44.4%).
BYU returns 109/379 carries (28.7%), 499/1996 yards (25.0%), and 4/18 TDs (22.2%)

Receiving yards:
Utah returns 107/212 catches (50.5%), 1319/2817 yards (46.8%), and 8/15 TDs (53.3%)
BYU returns 105/245 catches (42.8%), 1121/2573 yards (43.6%), and 6/15 TDs (40%).

Now your OL will be more experienced than ours, but I'm not sure Utah takes a big enough step back that BYU's skill players can close these gaps on offense. And it's Utah, so I'll always believe our defense better than yours, and I know our DL will be better than yours.