Nobody's said any one reliever will fix the whole pen. That player doesn't exist. A great loogy is, or can be, part of the solution.
How would you rate Perez? Maybe not great, but pretty darned good. An upgrade would have to be a very low priority, because you really can't use two roster spots for guys who can only get left handed batters out. So if we acquired Blevins we'd have to unload Perez. We'd have to pay him. And for what? A slight improvement in that role?
