I don't think that there should be 6th WR. A 5th isn't necessary as a receiver. Certainly, a 6th WR isn't.
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I do understand labeling a player a 6th WR if he is our primary punt returner, kick returner, or a key protector for the punt returner, or even a key ST specialist on several ST unit. For, that is simple for labeling. The player described here is a STer as Slater was.
In the best case, it is the FIFTH WR who is important as a STer and is an emergency #4 WR of there is an injury.
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Fo me, we are fine on the 53 with
Doubs, Brown/Pop, Hollins, Boutte, Williams
or with another replacement for Pop if we don't trade for Brown.
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And yes, I do want to carry the usual 25th player on Offense who is key in one of the ST roles.
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IMO, Chism isn't that player. If he is a future #3 WR and currently a #5, then he will win that position in camp or not.