I'm reluctant to be too critical of Stephens because he's still very inexperienced at Premier League level and it's not his fault that he's been thrown in at the deep end, but boy does he need to learn fast.
He has been bullied by both Llorente and Carroll. Forget about the quality of Obiang's through ball, there is no way he should be letting Carroll get inside him for the first goal yesterday. He didn't cover himself in glory for the third goal either. Granted it was Romeu who gave the free kick away, but it was Stephens' initial indecision that he was trying to make up for.
I don't think he was helped by Yoshida at all yesterday, who should be stepping up to the plate as the experienced centre half. I thought their positioning was largely poor and the gap between them was too wide on several occasions, as seen by the equaliser.
Technically he looks decent on the ball and, as others have said, I like that he has the confidence to try and bring the ball out from the back at times. I can see why the club have supposedly earmarked him as a defensive midfielder, but I'm not convinced with him as a centre back in the long term.