The answer is yes, acquire Derrick Henry.
He is essentially one of 4-5 league winner handcuffs , where if the right situation occurs, he gives you tremendous upside if Murray goes down. Simply put, he can be a RB1. One of the most valuable cuffs there is.
But he correct play is, do not drop John Brown, but drop Randall Cobb. Brown is primed for a bit of a resurgence. Even though they may have similar ceilings, John Brown has a clearer path to more steady work whereas Cobb would need an injury to someone in front of him (Nelson or Adams) and even that isn't a guarantee.
When Nelson went down year before last, Cobb severely disappointed owners. I think its close, but I drop Cobb. (the one thing that may get me to debate that, is you own Nelson as well, so its another "handcuff in a sense, albeit at WR)
Either way, RUN TO THE WIRE and acquire Henry, no matter who you drop.
TZM