I always wonder how such a downfall comes about. He can’t have forgotten how to kick in the last few years. How can he destroy the Premier League for Watford’s standards a few years ago, only to now not be good enough for the second division? There were no major injuries
In the case of Dele Alli, there were mental problems that are difficult to see from the outside.
Otherwise there could also be other reasons. Do you feel comfortable in the club? Do you have a trainer you trust? How flexible are you tactically deployable?
The latter in particular is something that can always cause fluctuations in a player’s performance. Just take someone like Werner as an example, who looked world-class in his first RB time with a system that was perfectly tailored to him and a man next to him who complemented him, only to now be completely left out at the same club because a different style of play with a different formation and a different personnel prevails and Werner is not flexible enough to adapt to a different system.
On the other hand, you can name players like Alaba, who have shown their class in a wide variety of roles and even in a variety of positions. There are such and such.
I can’t say what exactly applies to Dennis, but there are many more reasons than just injuries/illnesses why a career can take such a hitch.