The Souza / McLouth / Hairston decision is not Williams's. It's Rizzo who makes that decision. Probably what Rizzo was thinking was that with Harper then ALR down, he was going to have to give one younger player, Moore, full-time duty at first and that his preferred solution to left field would be to give the veteran
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a first shot at working out while Souza could play full time in AAA and get PAs and more outfield experience.
We may be getting to the point where you could see Rizzo moving Hairston (not McLouth) off the roster and bring up Souza full time to play left. McLouth can back up all 3 OF positions and his defense is useful even if his offense isn't (although monday he made nice contact twice). Hairston can only be hidden in left, not well, and his bat is useful only against lefties. Frandsen has been maybe closer to adequate than hairston this year, so that covers your back up righty bat.
We've lost a bit of flexibility for call ups when both Kobernus and Perez went on the 60 day DL.