There are three separate issues:
1) are our players, individually, good enough for the Premiership?
2) are our players deployed to best advantage?
3) are our players motivated/de-motivated?
My concern is NOT (1). Obviously losing both Demps. and Dembele at the end of the transfer window weakened our squad, but we still have plenty of decent players including, arguably, the best back four we have had for a while as well as the best striker we've had since Saha went to Man United and arguably before that. With two key midfield players rarely able to play due to injuries, we are weak there, but one good signing would substantially remedy that (and wouldn't of course have been necessary had our management been more on the ball).
I believe the problem lies not with the calibre of our players but in (2) and (3) and responsibility for that lies squarely with the manager. Jol may have a vision about the football he would like to see us play, but what delivers results is using the players we've actually got effectively. Our defence, excellent under Roy and Hughes, now looks fragile and no side does well in the Premiership with a fragile defence. And as regards man management and motivation, it has been obvious from very early on that Jol is a complete lemon.