It's been suggested the former Bayer Leverkusen star doesn't fit within Lampard's system.
But the manager says: "Kai has played various positions in career, as number nine, number eight and a number 10.
"He played lots off the right for Leverkusen and I have my own vision going forward.
"The key now is getting him fit.
"Kai, with the situation we are in we aren't in a position where we can wait. We need to get him fit, and it'll become very clear why he's here."