England's dominant win by an innings and 78 runs in the fourth Test at Nottingham's Trent Bridge ground yesterday gave them an unbeatable 3-1 lead in the five-match series.
Asked how much responsibility he and fellow former Australia international Marsh, the on-tour selector, bore for the series defeat, Lehmann told reporters: "A lot. As a coach and selectors we've got things wrong at certain times, there's no doubt about that."
Lehmann said a particular mistake at Trent Bridge had been for Australia to abandon their longstanding policy of fielding five bowlers, with all-rounder Mitchell Marsh dropped to make way for older brother Shaun, a top-order batsman.
England, with Joe Root scoring 130, piled up 391 for nine declared in reply against an Australia attack who, in the circumstances, needed all the bowling they could muster.
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"I spoke to Rodney and we wanted the extra batter and spoke to the captain about it in this particular game, but we've always wanted five bowlers, so we probably got that selection wrong," Lehmann said.
"We're really strong on having five bowlers and we didn't do that this game."
Defeat at Trent Bridge condemned Australia to their fourth successive Ashes series loss in England, a run stretching back 14 years.
A feature of the series so far has been the inability of Australia's batsmen to cope with the swinging and seaming ball on typical English pitches that aid lateral movement.
There have been accusations that too many Australia batsmen are 'flat-track bullies', unwilling as much as unable to alter the methods that serve them so well on flatter home surfaces.