Well, if you currently have Acevedo, Nelson and Borowski all on your roster, I would drop some of them to get the permanent closers off the wire. Acevedo won't be closing now that Rivera is back from the DL so probably has little value. Alfonseca is back early next week probably, and will likely get the closer's job back, giving Borowski little value. It's unclear when Sasaki will be back, but the closer job in Seattle is currently a platoon with Nelson and Rhodes, meaning that Nelson even right now likely isn't much more useful than a permanent closer.
Looper and Carter have both looked solid so far. Looper does have the advantage that he plays for a team likely to win more games than Tampa. Also, because of the way Florida plays, relying on speed and "small ball", they are likely to have more save opportunities than many teams. I don't think Anderson is that bad of an option either, because Detroit has to win 40 games and they won't be blowing people out, meaning Anderson is likely to get 25-30 saves. I wouldn't touch Escobar with a 10 foot pole right now; he is getting shelled in save and non-save situations and the Jays are likely to trade him to a contender to be a setup man as soon as he gets it together. In the meantime, he will be nothing more than an ERA and WHIP drain.
If you currently have Acevedo, Borowski and Nelson on your roster, I would dump all three of them for Looper, Carter, and Anderson.
If you can only pick up one or two of them, I would do so in this order of preference:
I only have one open spot; I had been using the others on a rotating basis as each had value for a finite period of time. But since there are two semi-solid, defined closers available, it seemed like the right move. Nelson was just the most recent move for me (as Alfonseca is due back).
I took Carter now and will consider Looper once he clears waivers. I am not wasting my WW position on him.
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I would use the waiver pick on Looper. He has been very good and looks to be finally ready to fulfill all that potential everyone thought he had. The fact that he was dropped is actually pretty amazing
In my opinion I would go with Looper. And as much as I hate to say this because I am a huge Billy Koch fan, you might also look at Marte if he is a free agent. As stated in other posts, Manuel has no patience with relievers. Just a hunch.