I never thought there would be a Barca team last in La Liga in goals allowed, this far into a season.
This team is much less likely than almost any other team to fire a manager during the season. However, that does not mean never. As of now, there just is not much to lose by giving a new manager a try.
But, whereas a new manager now does seem necessary, it is impossible for any new manager to instantly, magically transform this squad into a contender for the triple. Too many players are just not good enough, or have gone stale/too old. Worse than that, the squad just does not play well together. The fit, the teamwork, not good at all.
Some things will likely improve as this season progresses, with or without Valverde. Most obviously, with Messi rejoining the lineup full time, there will be more goals and more wins. Griezmann and De Jong will integrate better, due to their inherent quality. The squad members who have played the worst will stop getting playing time.