That's what happened to him: He got injured and subsequently, got fat.
Like others have said though, it's a real shame because when he first came onto the scene I thought he looked a great talent and would have loved for us to sign him at that point. Not sure exactly what's happened since (beyond just the injuries) but I remember being shocked when I first saw the below picture.
If he ever wants to get his career back on track though, he has to leave City. Not a chance in hell will he get a single minute there now.