Ok, you are his fan and I know that you watched him more.
You probably have more infos than us.
I have a feeling for now that Deulo can take the ball and create a dangerous chance WHENEVER he wants.
And I have a feeling that with improved/higher Stamina, he would be able to do a mess in the opponents box way more often.
Currently he tries only 2-3 solo actions per half, but again, I have a feeling that with more stamina, he would be able to do the same 5-6-7 times per half.
As if he makes a solo run and then he shuts down for a several minutes to recharge his batteries.
Which is a somewhat weird because he is only 23 years old.
He should be able to run like a robot all over the field in that age (like Sergi roberto).
My theory is, from watching him extensively over the years, is this. He's always been used to being the de facto star man on his team, similar to Messi now, but obviously on a lot lower level. The same way Messi refuses to defend most of the time so he can focus on scoring, is what Deu had been used to for most of his life. But since starting in top flight football, suddenly every coach he had would complain mainly about one thing, as his selfishness, end product, teamwork, etc were all things they thought they themselves could fix (it seems), and that is his defending. Taking a player who only knows attacking and thrusting him into a new level where he has to run a lot more is going to cause stamina issues. Even at Everton, his stamina issues only started becoming an issue from what I could tell when he was making last man tackles and sprinting to cover for his fullback, which imo was kind of overdoing it, but I get he was desperate at the time. At Milan he was trying his good movements as often as he could while also making a lot of effort to defend and he fizzled out as his time there went on, causing the fans to sour on him. Some would say he failed at Milan because of this but I don't think so, a player like him can't run as much as he was while also starting every game and playing the full 90.
Neymar was in a similar boat but you didn't see nearly the same level of defensive effort. He'd track back and be there if needed but his defensive work rate was never anything to write home about. He focused primarily on his dribbles that usually led to absolutely nothing and any way he could possibly get the ball to Messi. Comparisons to Pedro going on in this thread is a can of worms I didn't want to open, but he was never a particularly skilled forward, he just made his way up through an incredible amount of hard work. He defended harder than our defenders and always went in as hard as he could (Even if that became super ineffective after MVP ended) in the attack, and that had always been his style, so his stamina was always going to be much better.
Overall, I think Deu's stamina problems are down to his incredible skill as an attacker at an early age. It's part of why I never wanted him as anything more than a backup (Okay, except for when I was a La Masia worshipping fool and thought every talent should start for us eventually, lol), although I do think it can be improved drastically if he's given enough minutes, because his stamina issues seem to be more fitness related, than genetic (Like Rakitic).