To me, Thornton is WAY better than Fields. They are talking about cutting Fields anyway.
Toronto doesnt have a true SG right now. Ross is a Sf and Louis Williams is tiny. If they acquired Thornton, they would be able to play Ross some at Sf. Their current backup SF, is also a tweener - James Johnson can play both forward spots.
Let Powell start the season in the D-league. On Dec 16th, make the trade. Then sign Powell to be the 15th man. You will be 22 games into the schedule. Maybe by then Rondo will be back in full swing. Maybe Thornton will have played well enough that we could get more for him.
Its just a thought. If Powell gets cut and someone else signs him, its not the end of the world. Maybe use Bynum and Thornton for the first 1/4 of the season, especially that first week or two as Rondo comes back. After that - with Rondo, Smart, Bradley, Turner around - there will be no need for them.