that's a very incomplete statement. He is empire state building long and in his second year in the NBA : he will fumble the majority of poorly drawn passes that are thrown too low, he needs the ball to be passed to the proper areas (minimum is waist high, ok being navel high and ideal being breast to top of the head high).
Second lots of observers tend to forget that he plays in the most crowded area in the court. There are arms and legs and bodies everywhere there plus he is like a gigantic spider with his limbs. This means that you are asking a freaking albatros standing in a phone booth to catch a ball he sees at the very last second (because of all those arms and legs around). If you want him to catch that ball, drop it be high enough. Contrary to a guy like Asik he almost always has his hands opened wide and high, where they are supposed to be when you play in the paint. If a guard turning a corner or attacking the basket drops it at the proper height, his fumble rate is very low.
Then there is the issue of getting the balls stripped before he can go up, which is an issue of strength and experience (ball protection).
Between poor passes (throwing quality passes to the big man is the guards's job, not his !) and stripped balls, we might get the impression he fumbles lots of passes, but he doesn't. decent enough passes he catches at a perfectly solid rate.
