I think Dallas will consider what gives the team its best lineup but lean toward keeping Lee at the WLB spot unless no one else can handle the MLB spot.
We only have three linebackers taller than 6-2 (McClain, Wilber and Brandon Hepburn) and four who weigh more than 240 (McClain, Wilber, Damien Wilson, Jaylon Smith and Derek Akunne). Lee is not one of them, so while playing the middle is something he can do, he's really not built to physically handle the position.
Hitchens and Durant really aren't either, though, so it will be interesting to see what the coaches decide to do. The best fit sizewise right now might be Damien Wilson, who is listed at 245 even though he's just 6-1, but he might not be one of our top three choices.
I also believe some combination of Lee, Durant and Hitchens will see the most playing time among the linebackers, but we'll see how Dallas mixes and matches them. There could be times when Lee is in the middle, Durant at Will and Hitchens at Sam and other times when Hitchens or Durant is in the middle. The versatility of these three could really benefit the corps' ability to change it up.