Players retain responsibility for their own performances, but that doesn't remove the coach/manager's responsibility for improving the players and the team.
If you think it doesn't, then what is he actually bringing to the party?
If the answer to “Given the players we've got, what can the manager do to make this team better?” is essentially “not much, because the players simply aren't good enough”, then fair enough. That may be a legitimate assessment of the squad he has built.
But it also raises a pretty obvious question: what is the manager actually contributing to improving the situation?
A manager can't turn poor players into world-beaters. But coaching is supposed to improve what you have, tactically, individually and collectively. It's supposed to find ways of getting more out of players, develop them, solve problems and make the team better than the sum of its parts.
That's the issue when “the squad isn't good enough” becomes the entire defence of the manager's performance, particularly when we're talking about a squad where he signed the majority of the players to fit his own philosophy, which appears to prioritise experience over almost everything else.
You can't simultaneously claim the players aren't good enough and treat that as entirely separate from the responsibility of the person who selected and coached them.