Hearsay? almost every "new" Poch player has commented on his training intensity. The most damning is the interview with Dembele who specifically says he left Spurs because his body couldn't handle the training, he could the games but not the training (the fact that Poch didn't modify specifically to help Dembele last longer with us is not a good look).
Re Dier, he's a lot heavier than the earlier version, and whether it's his illness or injuries he's definitely lost something in his first step, just seems like one of those players who's physically lost it earlier than expected.
Latest idea from pundits/etc, is for him to return to CB role, not sure how being slower and often behind the game translates into fit in as CB. He is a priority to replace in summer, it would take a massive turnaround in form and physical output to see him play at this level consistently again.