I think we need to realise that our squad is pretty bad. It was falling apart before Koeman joined and although we've spent a decent amount, a lot of players that we've brought in just aren't that great. Last season Rom made us look better than we were. Coleman is our best player, after that no one really stands out as being top class.
I'm not saying that a different manager couldn't get more out of them but I don't think Koeman should take all of the blame.
Most of the players today were signed by him, though. The reason our squad is bad is that money has been wasted on players that aren't good enough in positions we didn't need to strengthen. The buck stops with the manager.