The simple fact is Jadeja got hit on the helmet, the Indian physio did a test and said he had concussion, so after that they are allowed a sub who can bowl.
Should he have had a test on the pitch when he got hit, yes. Would he have come off then, maybe.
But as soon as the physio says he has concussion it's end of story because whats David Boon going to do call him a liar and tell Jadeja to get on the park! Of course not.
Maybe the Indians have bent the rules, maybe Jadeja had delayed concussion, only they will know.