This is fair enough. I don't agree totally. I think Carrick looked fine there this season, and his positioning was still an asset.
And I don't think Blind's perfectly suited to it (especially as the last line ahead of the centre backs).
I think the bigger issue is that we play one of our central midfielders inside the opponents box. So when we lose the ball we're not playing Blind behind two central midfielders but rather Blind as a defensive midfielder with only Herrera to help him out. Herrera himself also likes pushing forward so very often we have Blind more or less alone as a central midfielder while Herrera works his magic with Mata and Fellaini is inside the box.
Ideally when Blind starts the press, the other two central midfielders covers the inevitable runs from the opponents to cut off the passing options.
Blind has done his job of aggressively intercepting to initiate pressure really well and he had the most interceptions on the opponents half I believe in the top leagues which shows that.
The problem is that there is no way Fellaini will track his man on the counter in case the initial pressure isn't working. The central midfield hasn't been working too great, that is a fact. But I would like to see Blind playing with a defensively better central midfield before making a call on if he can do it at the desired level or not.
I do agree that it has been pretty ugly at times seeing Blind aggressively trying to intercept a ball when Fellaini is inside the box and Herrera is close to it or out wide right. When it doesn't work it means there is going to be a good chance for the opponents and trying to play passively is probably a better idea. But like I said, the problem doesn't start there as hardly any defensive midfielder would look great when they are as exposed as ours has been due to that midfield composition.
Busquets looked terrible in the same situation for Barcelona earlier this season when their team wasn't clicking too well.