The reason he isn't playing has 100% to do with Matuidi's role as Sarri has suggested. When he's talking about symmetry he is talking about how Ronaldo is playing a free role and often isn't there on the LW and isn't really defending. Matuidi as a result is playing as a CM with responsibilities to cover the entire left flank. He did the same thing for France. Unless Rabiot can get into strong enough shape to do that as well, Matuidi will be favoured. Matuidi will play there until either 1) Ronaldo leaves 2) Sarri plays Ronaldo centrally. He didn't have this problem with Napoli bc Insigne & Callejon both defend and would stay on their flanks.
Matuidi being favoured for how he readily made fit he is characterisim wise and his running capabiliies is one thing (basically this Juve's Allan for Sarri). But if it was just about Matuidi being prefered over him, then Rabiot would gotten a sub appearance as vice Khedira or Pjanic, but he hasnt. Can, Bentancur, and now the previously injured all year Ramsey is getting subs and a start. That tells me it has little to do with just Matuidi being preferred (thats a challenge for a later day), Rabiot not getting a single minute on the pitch still has to be bigger then that.
So defenitely something is wrong in terms of how prepared he is to play. Sometimes we have to take things at face value, the coach is litterally telling us he isnt ready and in good shape. Whatever he is doing in training is not convincing them. But still early days. Needs to get going, because no one will wait around to give him chance unless he is fully prepared for it.