Rondo has been disappointing, and his defensive effort has definitely fallen off after the first couple weeks of the season.
However, this team has bigger issues.
Also, I expect Bradley taking over the on-ball defense to be a HUGE plus for this team, and Rondo in particular. Rondo is very overhyped as an on-ball defender. While he can torment guys occasionally, his strength is as a floater in the team defense, reaching in, hunting down rebounds, etc. With Bradley covering the other teams ball handler, it frees up Rondo to do the things he is best at, which is what made the team so dangerous down the stretch last year.