If he's not FULLY - and I mean fully as in stronger than he ever has been before healed, then I wouldn't care if he got no carries at all this year.
I agree...the pick already makes me nervous (taking damaged goods that high in the draft is like buying a used, older luxury vehicle...it might work out as a bargain and give you a Jag or a Benz for the price of a Kia, but it also might end up costing you huge in repairs and rentals as stop gaps - in this case trips to the DL and missed seasons); but if he is not completely healed and recovered, then keep him off the field until he is.
If somehow Gurley is fully healed, I still would avoid trying to feed him too many carries and have him absorb too many hits. Maybe he really will be the reincarnation of Herchel Walker and just too good to keep off the field, but with Mason available, neither back should go much over 240 carries (15 touches per game) unless the other is injured or unproductive.
If all backs are healthy:
15 carries - Gurley
15 carries - Mason
5 carries - Austin
5 carries - others
If Gurley is not healthy:
20 carries - Mason
10 carries - Cunningham
5 carries - Austin
5 carries - others
That is ~40 carries a game. 35-40 runs/30-35 passes is about the balance I think Fisher would like. He is not Ground Chuck after all, just the closest thing left in the modern NFL...LOL!
I still think an ACL injury in November is really pushing it too hard to play less than 12 months post surgery, but the doctors and coaches and Gurley will make that call.