The Reds were looking to reassert themselves in the League One promotion race after a defeat at Sunderland the previous time out. But they fell short against a well-oiled Wycombe side, who are causing surprises this season.
They were second best throughout and fell to Randell Williams’s second-half strike, which sees Stendel’s men fall six points adrift of the top two.
“I have no explanation for this performance,” said the Barnsley boss.
“I’m not sure we played that badly, but I don’t know why we played how we did. We knew Wycombe would be strong from long balls and set-pieces, but they scored from a set-piece.
“We knew they would stop us getting space, but we had the ball always on the halfway line and not enough near their goal.
“We needed more power and more speed in our attack. We did not have a high enough tempo. My impression was that we were tired, but I don’t know why.”
Wycombe enjoyed the best chance of the first half that fell to Alex Samuel, whose near-post flick from a Nathan Tyson cross flashed across the goal, and also called for a penalty in the 20th minute as McGeehan challenged Samuel, but Anthony Coggins waved away the protests.
Wycombe continued their dominance after the restart and took the lead in the 55th minute, Williams volleying through the crowd and past Barnsley’s Adam Davies.
Wycombe Wanderers: Henderson, McCarthy, Jombati, El-Abd, Jacobson, Onyedinma, Thompson, Gape, Randell Williams, Tyson, Samuel. Unused substitutes: Stewart, Mackail-Smith, Kashket, Cowan-Hall, Yates, Morris, Freeman.
Barnsley: Davies, Brown, Pinnock, Lindsay, Fryers (Thiam 76), Bahre (Potts 56), Mowatt, McGeehan, Moncur, Woodrow (Jordan Williams 54), Moore. Unused substitutes: Isgrove, Jackson, Ben Williams, Greatorex.
Referee: A Coggins (Oxfordshire).