Not a fan of this. If you are in rebuild mode, then I would be looking to get better prospects and ditch Lee. Just the trade alone without looking at your team, I still probably wouldn't do that deal. I am not very confident in how long Lee is going to remain a top notch SP, his body is breaking down and I would rather have someone else take that risk especially if you are rebuilding this team. Davis and Scherzer are far from sure things. I think Davis can be a good 1B but I am not sure if he will ever break in to the top 5 and Scherzer is a good SP but more of a #3/4/5 kind of guy. Giving up Fielder, I would see if you could make some moves for guys like Bruce, Justin Upton, Kershaw, or Rasmus. Basically guys that are still young and are on the verge of breaking out. Not easy by any means but I like the upside of these guys more than I like the upside of the guys in the trade above. I would target teams that are close to the top of the league, who would be willing to let go of some younger talent in the hopes of winning this year.
"In the beginning the Universe was created. This has made a lot of people very angry and has been widely regarded as a bad move."- Douglas Adams
If you are in rebuilding mode and wanting to trade Fielder and Beckett, I would be looking for some more young talent in return besides Scherzer. Like Maud said, shoot for Bruce, Rasmus, and others in that territory that have not yet broken out just yet and could have your team on the upswing this coming season.