All indications are that Jeffrey Marquez is being called up to replace the injured Jake Peavy on the roster. To answer the question, it's not entirely clear why. Barring a postponed game (possible considering the weather for tonight) or the White Sox deciding to go with Freddy Garcia on short rest (unlikely), they're going to need a starter on Sunday. Dan Hudson, who has pitched very well, albeit inefficiently, would be on five days rest and was scheduled to pitch that day anyway in the Futures Game. Carlos Torres, who has pitched pretty well, would be on regular rest.
It's possible - maybe even likely - that Marquez is here only until Sunday to provide an interim long relief option while Tony Pena recuperates. Marquez was scheduled to start today for Charlotte and thus is fully rested; Torres pitched last night and wouldn't be able to pitch for the next few days; similar story for Hudson. Therefore, it's Marquez by default and then he'll be replaced by Hudson.
Marquez hasn't been particularly impressive in AAA. He's thrown 94 innings, giving up 99 hits, walking 31 and striking out 54. That's fine for a mop up man - let's just hope that's all he's supposed to be. A contending team in a tight race needs to send out its best possible players. And that's Hudson on Sunday. [edit: Hudson is apparently at least coming up for Sunday.]