The Russian season ended and Chelsea signed Ivanovic.
Basically, the guy played a full season with Lokomotiv and signed for Chelsea while they were at half season. Conditionally he was way behind the others in Chelsea.
Ok, this certainly isn't the main reason why he didn't play but it's probably one of the reasons why he stood no chance against the Chelsea Cb's (Carvalho, Terry, Alex), who are superior to him anyway (atm, at least).
Another thing is that after he joined them every match for Chelsea was do or die. They were chasing ManU in the league and playing knock out's in CL.
Grant couldn't afford resting his main players and experimenting with Ivanovic.
He's a good defender, no doubts about that. The reason why he didn't play certainly isn't because he's no good.
He looks great in partnership with Vidic, but i'm yet to see him play for a great club against great opposition (actually, i saw him once in such match, against Portugal, and he's been terrific) and i can't form an opinion about him and say if he's a player we should sign or not.