He has some stuff to learn still, when he drifts into that position. There were several instances against Brentford when he became very static in the midfield area, presumably inexperienced in the movements an 8 can make to help his teammates; on one occasion in particular, Henderson outside of him was desperate for him to make the looping overlap run back out wide, that Henderson himself would have made for Trent in the same position, but Trent was entirely still, forcing Henderson to go back to Matip.