Personally I very much disagree Dack is our best player. He's had 1 good season at this level in his entire career, and that was playing behind a number 9 in Danny Graham who was a Premier League regular at his peak and had the experience and ability to complement Dack's game perfectly.
That classic number 10 position of Dack's arguably doesn't exist now with players of his type (ability but little pace) having to drop into a more all-round midfield role. Some players, like Harvey Elliott, have adapted to it. I think Dack is struggling with it.
There's also the issue that he's now torn his ACL in both knees. I always think back to the effect on Shearer with similar injuries, tore his ACL aged 22 and fully recovered, tore his ankle ligament aged 27 and it wiped out his pace and turned a 30-goal a season striker into a 20-goal one. Dack's tears were a few days before his 26th birthday, and at 27. I think they happened at an age that could have finished him at this level.
There were 9 matches at the end of last season following Dack's return to fitness against Bristol, and despite us being on a bad run, Mowbray didn't start him in any. That suggests this isn't a JDT thing.