Not sure where the defense reference was. Maybe the Herrera comment where his speed in 2018 and 2019 was noticeably lower but where that relates to defense is also borne out in the defensive stats (dWAR, various zone measurements). Herrera started out grading as an above average CF. He slipped to average after a few years and was below average in 2018 and 2019. Whether he can recover is probably related to his speed because he did show the instincts to be above average before and he should not theoretically be too old for the position.
Where are the other CF candidates defensively? First we must admit that the data on all of them is sketchy. I think the Phillies rank them Kingery>Quinn>Herrera. Quinn probably has the potential to be the best of the three but he makes too many mistakes and takes too many chances. But he is faster than Kingery and has a better arm so the tools are there. Both Kingery and Quinn probably are average defensive CFs these days.
Haseley is a little different in that he does not have the speed you want in CF but has good defensive instincts in general. This leads to his being above average in the corners (either as his arm seems decent even though the scouting reports did not like it) and below average in CF. There have been CFs with below average speed and good instincts. Jim Edmonds for one. He was probably not as good as his media reputation because of how they played up the diving catches that Andruw Jones would get easily, but he was still a solid above average in CF. Could Haseley become that? Sure. But it is hard to bet on that and I am not sure his offense is good enough to justify playing a lot of CF with what is now below average defense.
I still believe in the power of numbers and youth here. And in midseason trades if necessary. Through platooning and defensive substitutions we should be able to cobble together 2 WAR total in CF. Just not sure whether it will be Kingery, Quinn, Haseley, Herrera, or someone else. They all have flaws, but whomever is playing best, will get the most playing time. It is really important to get Kingery going because he is probably the best potential offensive player here (on realistic projections). If more than one of the players play well it is a good problem to have.