I don't believe Glenn is a key. If he performs then it will be fantastic, but I don't see him being a 'key' to this season.
You didn't say number 2 receiver. You said Easely. I'd agree that number 2 wide receiver is a 'key', but I fail to see how a player that has never played a down in the regular season and has never been a factor is a 'key' to the entire season.
Merriman, again, could be helpful or valuable pass rush depth, but hardly a 'key' to the season.
McGee is the only one of those I'd even consider a 'key'. When he's healthy, he's a very good, reliable corner. If he stays healthy, we'd have excellent secondary depth.
I think what you define as a 'key' is very different from my view that a 'key' to the season is something is *needed* to be successful.