I think in this case it could be that they have played a different style and he hasn't been a fit. I'm being hopeful in saying it, but it seems that he has had a decent career that has stalled for a few months because of that, and perhaps the Boro fans have short memories of his contribution made during their promotion year then into the premier season.
Of course that Boro team was famous for how Karanka was very defensive and they never scored, so it could be that his defensive qualities are a much bigger part of his game and so he couldn't get in on that basis.
Of course the main thing we need at present is a strong, focused, tidying-up central midfielder, who provides a defensive line in front of the back four and can feed the ball to our creative players. A bit of a Batty, in short.
Let's hope that what Forshaw might have been before, but hasn't been needed for recently at Boro, is just what we need - a much better and stronger version of O Kane - who doesn't go off for a wander with the ball and squander possession, but just wins it, and distributes to Saiz or Hernandez etc with accuracy.
If that's what he is and does, it could be a great signing.
Don't shoot me if it goes wrong, i'm just trying to see the thinking in signing him, from reading about his game, without ever having seen him play to my knowledge.