Um, er... um. This is the type of deal that can keep you awake at night.
With all other factors being equal, I think the return for Johnson is a bit too low here. However, I'm not too worried about injury to the Big Unit, and I expect Uribe to get out of his slump fairly soon - if you think differently, then take this recommendation with a grain of salt. Still, I think you can hold out for a higher offer.
Also, if you've already used a large chunk of your pitching allotment, dealing Johnson for two solid bats would look good (although Williams and Schmidt would be unnecessary in this case).