I trust Hays to play some CF.
Since 2020, he's been worth +2 DRS as a centerfielder in 274.2 innings at the position.
In that time, Mullins has been worth +15 DRS as a centerfielder in 4,044.1 innings.
I'm not saying he's a better defender than Mullins. I also know there are a lot of different and imperfect stats when it comes to defense. But I think it's promising that he's averaging +1 DRS every 137 innings as a CF, considering Mullins is averaging +1 DRS every 270.
I also think Mullins will heat back up.