It's very hard to measure the value of their contribution, but I for one think Capoue was one of our best players at the start of last season. Yes, I think he played better than Dier has in the first games.
The problem is more in the depht than in the quality or talent. Our midfield options are very unbalanced in terms of experience and defensive capabilities. I think Alli will become a much better player than Capoue and Stambouli, but probably not this season. More importantly however, he's a totally different player. You might say we've replaced three defensive midfielders with three attacking ones. Hence we're playing a centre back as dm.
I'm not unhappy with the players we've got, but our midfield cries out for a more defensive minded, and experienced midfielder.