Raul Jimenez struck the only goal of the game as Harry Wilson took advantage of a Scarles error to find the Mexican with a clever cross.
“I spoke to Ollie at the end on the pitch and told him just to keep his head up,” said Walker-Peters. “I've made plenty of mistakes. I know how it feels, but you come back stronger from them and he's got a long career ahead of him.
“But, like I said, he needs to keep his head up because we've got another game Tuesday and we need to win it.”
The fullback also said: “It's just one of those days. All the prep we put in throughout the week, everything went to plan and unfortunately we just weren't able to get it over the line.
“With this period, there are a lot of games so we've got a chance to put it right pretty quickly so we'll be looking to do that on Tuesday.”