Ki's a solid player. Good passing range, won't ever lose the ball. Keeps the team very tidy and the ball moving.
He was sensational for a season or two, because he seemed to have all of his technical qualities now, but also had non-stop hard running under him. He's lost a bit o that dynamic running capability now. Still, at least as much quality as Diame but gives us something different, Diame will be box to box allowing Shelvey to play deep-lying playmaker, Ki will be the solid deep-lying midfield sweeper who allows Shelvey to get more forward and control the attack.
Ki is like a Shelvey with a harder tackle and more running, but perhaps not as much creativity. I imagine Rafa being able to rotate Diame and Ki next to Shelvey very effectively depending on what kind of game-plan he has for each game.
Watch him, you might be underwhelmed now, but his quality will shine through over a season, mark my words.