Khedira is nothing special, absolutely.
He's a guy to fill in who is considered experienced and reliable, pretty much like Hernanes. Doesn't do anything specially well. Adds balance to the team and that's it. Can't really defend well, also can't really create.
And (surprise) he's always injured.
As a rotation player, and despite his absurdely high salaries (some of the biggest of the squad), his cost isn't that high because he came for free, but it isn't cheap either. I considered it a decent signing, but I wouldn't have made it, exactly because of his injury history.
If or when he comes back from the injury, I expect him to ride the bench, with Pogba and Pereyra as starters, along with Marchisio.
Pereyra is clearly a much better player.