You don't need bags of pace to be a good winger, it helps but there have been plenty of quick headless chickens playing on the wing and they soon get found out. You don't need to beat your man at all to be a good winger, you need to get a bit of space to put the ball in the box, if that's how you play or you need to be able to keep the ball under pressure to manipulate space for others.
Doku is quick enough and skilful enough to beat his man. The main issue with him is that he doesn't seem to know what to do after that. His awareness of what is happening around him, or what his intention with the ball is raises the biggest question mark over him. Sometimes he beats his man, then gets his head up and the defender gets back, then beats him again and puts a ball across. If he can learn to play with his head up a bit more he'll be miles better, or trust his instincts and do what he did against Villa - beat the man and put it across the box. That can come with more time on the pitch and people getting that connection. When De Bruyne got the ball, Haaland knew what he was going to do. I think one of the issues we've had with Haaland recently is that he keeps going for that back post run but we didn't have De Bruyne playing that ball - he needs to learn the near post run or dropping slightly deeper is more effective with Doku too.