I'd say they need to have a top 5 (or so) rushing attack, it doesn't mean they need a top 5 back.
Granted, it definitely helps but the last thing I want to see is the Bills think that acquiring McCoy and then saying, "we're done fixing the running game." This team still needs at least one more starter on the OL (at G) and they should still bring in a vet RT who can compete for a starting job. Do that and the running game has a chance of being fixed. Stop with this trade and it will be better, but not the dominant attack it needs to be to compensate for the QB.