The reason he played at RB was because we couldn't play out from the back well in the 1st half with Walker, Stones, Ota and Mendy. Stones and Ota didn't move the ball quickly enough to get up the pitch and Walker couldn't play it forward and when he did Bernardo also gave the ball away a lot. Pep had Garcia to try to find Foden along with Raheem to try to get a hold of the ball. He also didn't have Garcia go forward as much.
That said you can say it didn't work, but a lot had to do with the CB's lack of ability on the ball esp. Otamendi. The best CB partnership right now is Laporte and Garcia because of their ability to play it quickly. Then you have Walker as the RB. When you have that and you can sustain the attacks longer and have less defending to do.