He's still wearing that knee brace... knowing how much Chuck hates those things, I imagine it's still hurting or he wouldn't be wearing it. Not that it's an excuse, just a possible explanation.
This isn't new. His defense has been slipping for months. As others have mentioned, he's been figured out. If you can shoot the jumper from 15 feet you can score on him. If you have a back to the basket game from 12 then you can burn him (see Chris Kaman).
Chuck Hayes is gone. Adelman is not one to change the starting lineup very quickly but with the addition of Hill and Jeffries and the return of Yao plus a big in the draft along with Scola and Andersen, Hayes days are numbered. The rockets will just let Hayes walk. So the phase out begins. Hill ANdersen and jeffries will eat into the bulk of his minutes. Adelman is still trying to win but if Hayes is not definitely contributing to that that you will continue to see a decline in Hayes role.