The Eagles are just going to move on to Smallwood to fill Sproles' role.
In a Madden world, I would offer Lewis for Vinny Curry to bring some depth and stability to the edge. Curry is somewhat expendable with the Eagles signing Long and drafting Barnett.
In the real world, the Eagles know that a defensive line rotation of Graham, Cox, Jernigan, Long, Barnett and Curry alone can be good enough to steal a playoff win. You don't move that kind of quality DL depth for a RB.
A trade for a draft pick only makes sense in Belichick is already planning on moving on from Lewis and is supremely confident that White/Burkhead can fill his role. It worked last year with Collins, but, for the Patriots, having the extra RB depth may be more important.
The 3rd down RB is the most underrated cog in this offense. The late '00's offense sputtered any time Faulk got hurt, the 2013 offense sputtered when Shane Vareen was hurt, the 2015 offense grinded to a halt when Lewis was hurt. If you trade Lewis, and the injury bug hits the remaining RBs, this offense is toast.