My two cents on the whole tracking back-pace vs acceleration discussion: if a player has as much offensive contribution as Doku already has for City, then its a bit of an ungrateful task to expect him to constantly sprint the whole pitch and prevent opponents' attack. He is not a wingback, he is a wing forward. He should occassionally help out sure, but you don't see Madrid asking Vinicius to cover what is a vast amount of space in their own half, because you would just fatigue him too much to be effective in attack. When your effectiveness comes from physically very demanding constant sprints and physical duels, it can deplete your energy levels very quickly.
Of course, you can say well Doku should just work on his conditioning. He obviously should, as do everybody. But at some point you can't improve much more there unless you sacrifice something else, either weight or style of play. You don't want either with Doku imo. His explosive power comes for a huge part from his muscle mass; the kid's frame has grown a lot in the last two years constantly rehabbing from injuries. You can already see how crucial his strength is for his game the way he shields the ball. I know the tracking back is how City play and this is one of the biggest challenges he will have to overcome, learning to defend properly (the other being movement without the ball on attack; he needs to run between the channels occasionally and not just wait for the ball in his feet every time). But learning to defend properly means more in the head than the legs, so decision making and better understanding of space between him and his teammates.
TL DR, Doku has great pace as well, although not elite Mbappe/Walker type of pace. But because of his constant explosive actions on offense, he is too knackered to track back by sprinting long distances. This is not solvable by more conditioning training but by covering him more instead of expecting him to cover, and teaching him to figure out when he should track back and when he shouldn't. "Always" is just not gonna work unless you lose major offensive output from him.
