Barnsley’s grip on the second automatic promotion spot in League One was loosened after defeat at Burton Albion.
The Reds lost 3-1, while Sunderland won 2-1 at Rochdale to move level on 79 points with second-placed Barnsley, with two games in hand.
“It’s a disappointing result” said Stendel.
“But also the performance. I think that we fight for 90 or 95 minutes but it was not enough to win today.
“The goals were too easy for Burton. I don’t think they were much better than us but they were more effective. They scored and we created less chances to score.
“All of the pressure that we had we could not get in the chances to score and when we were defending as we did for the last two goal then you win no games.”
Barnsley fell behind to Allen’s fifth-minute goal at the Pirelli Stadium and Boyce doubled the Brewers’ lead with nine minutes left.
Cauley Woodrow’s 85th-minute penalty gave Daniel Stendel’s side hope, but Harness restored Burton’s two-goal advantage and sealed the win in the 89th minute.