(Reuters) - England's first tournament qualifying defeat in 10 years on Friday was not a cause for panic but the uncertainty over their best formation has given manager Gareth Southgate plenty to ponder ahead of Monday's Euro 2020 qualifier in Bulgaria.
Top of Group A with a far superior goal difference despite the 2-1 defeat away to the Czech Republic, England are still set to reach their sixth straight finals but any success at next year's tournament could come down to resolving tactical flaws.
During England's World Cup campaign last year, Southgate deployed a 3-5-2 system to great acclaim as his team conceded just four goals en route to the semi-finals in Russia.
However, partly to accommodate England's attacking talent, Southgate has evolved his formation into 4-3-3, modifying that further against the Czechs by playing Harry Kane, Raheem Sterling, Jadon Sancho and Mason Mount together from the start.
The problem for England in a lacklustre first-half display in Prague was the lack of a link in midfield, with holding midfielders Declan Rice and Jordan Henderson crowded out and unable to provide service to their four-man attack.
"They're having no impact on the game," former Manchester United midfielder Roy Keane said at halftime with the teams level at 1-1 after Harry Kane had put England ahead with a fifth-minute penalty that was cancelled out by Jakub Brabec.
Defenders Michael Keane and Harry Maguire were forced to make more passes with fewer options and that often resulted in long balls being easily dealt with by the Czechs, with England's switch to 4-3-3 in the second half also proving ineffective.
The Czechs still looked dangerous and grabbed a late winner through 30-year-old substitute Zdenek Ondrasek on his debut.
So should Southgate revert to a 3-5-2 system against the group's bottom side Bulgaria? Defensively England have looked much more vulnerable since the formation switch, having conceded three goals in a 5-3 win at home to Kosovo last month.
Czech Republic had 17 shots in Prague, the most England have faced in a qualifying match since March 2013.
Southgate can either still try to fit Kane, Sancho, Sterling and Mount -- not to mention Marcus Rashford and Callum Hudson-Odoi -- into the same team or sacrifice some of those players to bring solidity to England's formation once more.
If the manager feels he does not have the right players, especially in midfield, to deploy a 4-3-3 or 3-4-3 shape, bringing in Liverpool Trent Alexander-Arnold and Leicester City's Ben Chilwell as wing backs looks a viable option.
Alexander-Arnold had more assists (12) than any defender in a Premier League season in 2018/19 and only Manchester City's Belgium midfielder Kevin De Bruyne has created more chances in the current campaign, while Chilwell has played a crucial role in Leicester City's impressive start to the season.
This change would allow Southgate to deploy three centre-backs again, while giving England an extra attacking dimension.
"We've always maintained that there's a lot of work to be done to be a really top team," he said after Friday's loss.
"We didn't look any more solid, and we were poor at using the ball in the first half.
"We collectively have to accept the result but, of course, as the manager then I have got to accept that as well."
The performance against Bulgaria, with a system that ensures England do not look vulnerable at the back while retaining their attacking threat, is now as important as the result.
England remain on the brink of qualification for Euro 2020 but, with just two competitive games left after Monday's clash, time is running out for Southgate to find a tactical formula that could bring success at next year's tournament.
(Reporting by Peter Hall; editing by Ken Ferris)