City battled to draw with Atalanta in the Champions League after Walker had to go in goal for the closing stages of the match.
The full-back replaced Claudio Bravo after the substitute goalkeeper was sent off during the 1-1 draw in Milan.
Bravo was shown a straight red for a sliding tackle on Josip Ilicic outside the box, having replaced first-choice keeper Ederson at half-time.
Walker said afterwards: "Did I volunteer? I think it was a bit of both, I try and sometimes banter the keepers in training but I found you sometimes have to take two touches to make a save.
"We came here for a point or victory. To come away from here with a draw is a good point."