Graham Turner admitted Shrewsbury had lost focus after securing promotion last week as they were beaten 3-1 at AFC Wimbledon in League Two.
Luke Moore's double after Byron Harrison's opener was enough for the home side despite Shane Cansdell-Sherriff's second-half effort.
However, despite defeat Turner refused to be downhearted, with the Shrews heading into League One for the first time since 1997.
"With the big following we had here, we would have liked to have put on a better show than that and got a better result, but we paid the penalty for being sloppy," said Turner.
"The preparation was unprofessional and I'll take responsibility for that. If anything had been at stake I think it would have been different, we would have been better prepared. Second half we never got going, we've got to make sure it doesn't sour what we've done all season.
"We've had a great season irrespective of what happened, and when we get on the coach we can reflect on how well we've done and the celebrations shouldn't be soured in any way, shape or form by that performance and result."
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