Even with this info, I think the choice to bring Ake on, rather than Sergio Gomez (and move Cancelo to the right side) is strange to me.
Changing the formation with 10 minutes to go is not usually a successful tactic, especially when you are sending a player that has been off with his passing for most of the match up the pitch to be the deeplying playmaker.
In the 2nd half, Walker was playing alot more in a back 3, so in this sense the substitution didn't change our formation.
This information actually makes alot more sense to me because it made no sense to make this sub and keep 3 defenders back as we were doing 2nd half.