Btw, we do have a very close historical parallel for this situation.
In 2006 the New Orleans Saints drafted Reggie Bush high in the first round (1.2)
In 2017 the Carolina Panthers drafted CMC high in the first round (1.8)
Both Bush and McCartney are highly hyped, smaller, speedy guys.
The Saints had a long-serving veteran running back who was already the #1 back, Deuce McAllister.
The Panthers have a long-serving veteran running back who was already the #1 back, Jonathan Stewart.
Both the Saints and Panthers indicated that they intended to use their new back as a complementary back to their veteran back, ie as a change of pace and 3rd down back.
McAllister ran for 1,057 yards and 10 TDs in 2006.
Bush ran for 565 yards and 5 TDs and had 88 receptions for 742 yards and 2 TDs.
Given the very striking similarities of the situations, I expect broadly similar numbers to Bush for CMC this year.
I don't think Stewart will have as good a year as McAllister, so CMC might be able to run for more yards than Bush did, but I would be surprised if CMC gets as many receptions as (88) as Bush did that year.
I would expect about 600 yards on the ground for CMC and 60-70 receptions for about 600 yards. Not too shabby but not worthy of massive hype either.