It's not like he has an alternative. What else could he do? He could risk playing him, and being left with Pique and Puyol against sevilla.
Also, I think Tata wanted to give the players the chance to redeem themselves, and that's why Pique is playing.
FYI I don't care about Bartra starting or not, I was only discussing the justification used. You cant possibly justify not putting him in the match day squad.
IMO there is no risk in playing Bartra, you have to play your players at some point. The risk of Bartra, a young defender getting injured is almost nil. Lets entertain your possibility for a second - that Bartra risks getting injured. It is a lot more likely that it will happen if the first game back is in an intense game like Sevilla away as compared to a meek one like Sociedad at home. As for him not having alternatives, that's just untrue. He could've maybe even subbed him on for a few minutes today to get some match fitness. Or better yet, started him, considering he's the second best defender.
Again this is not about Bartra getting playing time, but just the bad excuses.