Pep should have swapped him and Foden on the flanks. Foden is good at opening behind the back, while Doku is good at 1 vs 1 situations. But on the left against Doku he played as one of the best defenders in 1 vs 1 situations, but who has problems controlling the space behind his back. So, it was necessary to change Foden and Doku as flanks. That's all.