England manager Gareth Southgate expressed his displeasure as his team scraped past Iceland 1-0 in their Nations League opener in Reykjavik on Saturday night.
Raheem Sterling scored the winner from the spot in the 91st minute, but England were fortunate to escape with all three points.
Walker had earlier been given the marching orders when he received a second yellow card for a tackle and Joe Gomez gave away a penalty moments after Sterling's strike, which was sent over the bar by Birkir Bjarnason.
"The sending off is then obviously a key moment because I felt we were even more in control in the second half," said Southgate as quoted by Sky Sports.
"It's very difficult to win games of football at any level if you're down to 10 men. That's a lesson we have to learn. It was an unnecessary red card."And then another lesson we have to learn, because to invite the ball as we did and defend it the way we did (for Iceland's penalty) was really poor play. In the end we get away with it."
"It kind of sums the week up really, it's been so difficult to piece everything together and so many challenges," he said.
"We leave here with a win, which was really important, some experience for some young players but obviously a bit flat because it feels that in the end we got away with it."We're going to have to assess everybody (ahead of the Denmark game) because it's not just the match sharpness (they're lacking) it's the actual training minutes for a lot of the guys as well.
"This would have taken a lot out of them. So we're going to have to assess freshness, what's the right approach for the game but at least we leave here with the win which gives us some momentum moving forward," he added.