I've heard this thrown around all day today and it's something I think maybe I don't fully understand— you be the judge.
Double coverage is a BIG part of every defensive gameplan (NFL or not) and is often dictated by the routes a coordinator calls more than anything else. It's unheard of to man-double cover anybody in the NFL. It's almost always a man-zone combination based on the routes being run.
Stevie Johnson is not the same type of receiver that Lee Evans is and he shouldn't change what he does because the team needs a "#1"... who ostensibly is a deep-threat who can separate on the deep fly.
Unless (and it begs the question) the Bills change how they use personnel based on this move— seems like a mistake to me—it's not going to effect how defenses play them.
Whoever fulfills Evans' role in the offense is a safe bet to see a lot of double coverage. By design, that shouldn't be Stevie Johnson.