Yikes. I can't say no enough to this.
Why would you create defensive deficiencies in two positions by replacing WMB with Youk at 3rd, and then putting WMB out in LF?? That doesn't make any sense.
Youkilis is just going to have to understand he's going to be a backup player at 1st and 3rd. I know that you're not going to build value for him that way, but what you do is you have him rehab in AAA for a couple of weeks and let him build value there.
As much as some people don't understand it, GM's don't look at stats as much as you'd like to think. They rely a whole lot more on their scouts, and if they're scouting Youk on his rehab assignment and the reports are saying "He looks healthy, hitting to all fields, not expanding the zone" etc, then GM's are going to still pull the trigger on a deal.
His injury history and concern is not going to just go away if he has a healthy 2-3 weeks. You hope that Youk has a healthy and productive rehab stint, and that he looks good in the eyes of scouts. If he can do that, then you've got a chip. If not, you let him rehab until he gets right, and then see what kind of offers are on the table.