England manager Gareth Southgate is considering returning to the three-man defence that helped his side reach the 2018 World Cup semi-finals.
Group A leaders England face Bulgaria in a Euro 2020 qualifier on Monday (19:45 BST), three days after being beaten 2-1 in the Czech Republic.
Defensive errors were punished in their first loss in a qualifier since 2009.
"We're very aware and assessing those things," said Southgate on a potential change in defensive tactics.
Harry Maguire, John Stones and Kyle Walker formed the back three at the tournament in Russia.
However, Stones is unavailable through injury, while Manchester City team-mate Walker was not been included in the squad.
"We are in a difficult situation in terms of the players that have played international football with us in that area of the pitch and who is playing regularly for their clubs," added Southgate.
"That's part of the reason we didn't do that [a back three against the Czech Republic]."
Asked if there is enough time to make a change to his system before Euro 2020, Southgate said: "I think so. I think players take in tactical concepts very quickly, especially top players.
"We have to keep reflecting on what's working and what's possible, and learn from every experience we have."
Poor club form at fault?
Manchester United defender Maguire, who was partnered by Everton centre-back Michael Keane in Prague, believes some of the England players' poor club form could have contributed to the below-par performance against the Czech Republic.
United are 12th in the Premier League after a disappointing start to the season, while the Toffees have lost five of their opening eight games.
Asked if there was a lack of confidence among the England defence, Maguire replied: "Yes, maybe.
"We bring our club football into international level. It was a difficult game. Obviously you are wanting to develop partnerships. Sometimes at international level you can't really do that.
"But there's no excuses. We need to be better than Friday night. We need to be better going forward. We need to be better on the ball. And we certainly need to be better at keeping the ball out of the net."