The Tykes had already sealed a top-six spot but missed the chance to climb the table with a sloppy display in the 2-0 defeat at Deepdale.
With potential play-off opponents Brentford and Swansea impressing with victories, Ismael said: “We didn’t put in the performance we wanted and it’s a big disappointment.
“It’s a typical case that when we reached our goal last week the pressure came down and we need to raise that.
“It’s a warning for everyone that we need to refocus and come back to our strength and intensity. The good thing is we will refocus and know what we have to do and find the right solution against Norwich.
“The training week was good and the intensity was there but at 3pm the guys switched off. We deserve our position but when you win it becomes an addiction so we’re disappointed.”
Ismael’s side have won 13 of their last 18 league matches and will focus on an unlikely Premier League push when the play-offs begin in a fortnight.
Jordan Storey poached the opener in the 38th minute after Ryan Ledson’s corner caused a scramble. Storey turned provider with a flick on for Ched Evans, who smartly finished.
There were late chances for Alex Mowatt and substitute Jasper Moon for Ismael’s side.
Preston North End: Iversen, Storey, Lindsay, Hughes, van den Berg; Ledson, Whiteman (Potts 82), Cunningham, Browne, Evans (Bayliss 90); Barkhuizen (Jakobsen 70). Unused substitutes: Molumby, Huntington, Maguire, Hudson, Sinclair, Gordon.
Barnsley: Collins, Sollbauer (Sibbick 46), Helik, Andersen; Brittain, Palmer (Moon 46), Mowatt, Styles (Jordan Williams 84); Chaplin (Morris 46), Dike, Woodrow (Adeboyejo 64).Unused substitutes:Walton, Oduor, Halme, Thompson.
Refeeree: M Donohue (Lancashire).
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