when we are defending with 3 at the back then we really have to have a midfielder sitting in front of the back 3 to stop them from playing there.
for the majority of the season it has been Pack filling that role, and he's pretty much the only senior player who I'd want doing it.
if you asked ralls or bacuna to do that role then you'd lose a lot of their game, and there are question marks over whether they would be good enough defensively, likewise vaulks.

pack is positionally disciplined, wins a lot of aerial balls and is a good passer of the ball.
on the down side though i don't think he's the quickest across the pitch, and he doesn't move the ball that quickly either sometimes.

to improve the defence then perhaps we need someone who can do the role pack has been doing but also be fast enough to close down more threats in that area. maybe Wintle is that man in the longer term, as he seems to do a fair amount of defensive work at Blackpool.