And that interval dressing down did the trick, as the Reds scored two unanswered goals after the resumption to claim a 3-2 victory which consolidated their place in the Championship's top-six.
Having taken an early lead through defender Michal Helik, the South Yorkshiremen then proceeded to switch off and found themselves 2-1 down by the break.
But, second-half efforts from Dominik Frieser and substitute Carlton Morris completed a second-half turnaround which gives the Oakwell club some breathing space in the division's final play-off berth.
"We did not start the game well, I think [because of] the tiredness, the travel to Bournemouth. We felt that we were not in the game," Ismael said.
"After our goal we had a lot of control, a lot of good situations, chances to score again, but then after that we made two idiotic, silly mistakes.
"Bournemouth were at that time dead on the pitch and we let them back in the game. The second goal, especially, just before half-time - you have to avoid that.
"We made the changes at half-time to give another impact in the game, to give the signal to the guys that we are here to win that we don't travel five hours here and five hours back to lose the game.
"It was the first time I was really angry. I was a little bit loud in the dressing room. I just told the guys we waste our time if we play like that and we ruin the good work we have done.
"So, congratulations to the guys for the second half, especially the bench for making another impact. It was a big performance."
Barnsley took the lead at Dean Court after 16 minutes when Mads Andersen met Alex Mowatt's left-wing corner at the near post and helped it across goal for Helik to volley in from barely a yard out.
Goals from Arnaut Danjuma and Dominic Solanke turned the game on its head, though the Reds were back on terms just after the hour-mark courtesy of Frieser's instinctive finish from Cauley Woodrow's low, deflected cross from the right.
Morris then decided matters, climbing at the far post to head home a Mowatt free-kick 10 minutes from time.
Victory means that Barnsley are now unbeaten in 11 matches and ensures they stay sixth in the table, five points clear of seventh-placed Bournemouth.