More to do with back issue. If you noticed the way he was stretching prior to coming off, it looked back pain to me.
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Whilst Stephens is probably the best/one of the best at championship level, he has IMO, surprisingly struggled so far (other than the defence splitting pass to Knocky).
This may be because of Propper (who does seem very similar) or because as you say he has been surprised by the pace.
Whilst...
I think potentially he is great.
The trouble is he has had a bit of a shock regarding the physicality of matches over here, a few times I've seen him getting frustrated with match officials when he has been barged over. The other thing is that our midfield are very slow moving the ball to him...