In view of the recent discussion over possible options for the Brewers to go out and get a SS in a trade from other ML rosters (which was fairly depressing), I figured I'd look to see if there were any viable options at the AAA level that might be possible solutions. This is entirely based on a very cursory research, so please feel free to add any actual, relevant, more-than-you-get-from-15-minutes-on-Fangraphs information.
Remember, the important metric here is just that they need to be BTY - better than Yuni!
Cody Ransom (AAA Diamondbacks) - He's old (35) and below average at SS defensively (-9.5 UZR/150 in only 326 innings in the majors - 7 Total Zone Runs above average in 948 minor league games at SS), so why is he here? Well, he'd probably be BTY. He's mashing up the PCL right now - 18 HRs, .990 OPS, .602 SLG. He hasn't shown the ability to hit for average much (career .247 in the minors, .227 in limited ML time), but he does have pop, take more walks than Yuni (not really an achievement, but Ransom has a BB% of around 10% or so compared to Yuni's awesome 3.4% career rate), and probably has more upside offensively than Josh Wilson for this year. Furthermore, he's probably available - he's obviously not a long-term prospect for the DBacks, and they have Stephen Drew set at SS. I would imagine that the Crew could get him for cheap (possibly just cash considerations), and he'd be a better option to platoon at SS with Counsell than Wilson, most likely.
Chase D'Arnaud (AAA Pirates) - He's been steadily climbing up the Pirates system, and it's probably more likely that Ronny Cedeno is the available SS at ML/AAA for the Pirates, but he'd be worth kicking the tires on. He's had a wOBA above .330 at every level of the minors, and his current line in AAA is.287/.350/.430 with a .357 wOBA. The few scouting reports I took the time to read seemed to say that he's average or better in the field, so if he could be had without giving up anything major, he'd be an interesting guy to have. Probably not likely that he's available, though, because I would imagine the Pirates see him in the future instead of Cedeno.
Luis Valbuena (AAA Indians) - He's absolutely mashed at AAA the past 3 years (wOBA of .424, .445, and .370), but he was pretty bad last year for the Indians at the ML level (.193/.273/.258 with bad defense mostly at 2B). It would take some scouting and the defense would still be bad (especially at SS), but he probably has more upside than Yuni and he might be available for cheap - the Indians have Asdrubal Cabrera entrenched at SS, Orlando Cabrera and Adam Everett have played most of the 2B and the more highly touted Cord Phelps recently got a call-up as well. Again, not a great option, but there's maybe a 50/50 shot that he's BTY with some upside.