Robins had to field a much-changed side to the one that drew at high-flying Cardiff eight days ago, and Robins felt it was a contributory factor in defeat.
He said: “We’ve got half the side out and it cost us. When we’ve got players out, that’s when we lose discipline.
“We had the midfield out and we had to use players who haven’t played a lot of football recently.
“We can hopefully now get some of the players fit during the international break.”
Defeat ended Barnsley’s three-match unbeaten run, and they have won just once in their last nine outings.
Robins added: “The ball was up in the air a lot and that’s not what I’ve asked them to do.
“The goal would have been a comedy if it wasn’t so disappointing. The one time when Nathan (Doyle) had to boot the ball clear, he didn’t do it.
“Our best two chances fell to Martin Devaney and he was slightly unfortunate to not put either of them away.”
Reading manager Brian McDermott hailed the impact of substitute Mathieu Manset after his goal earned the Royals their win.
Manset netted the winner within five minutes of his second-half introduction as the visitors finally found a way past Barnsley goalkeeper Luke Steele.
And the Frenchman, whose goal came with 19 minutes left, earned the praise of his boss.
“When the ball goes near Mathieu, he comes alive,” said McDermott. “He’ll be a good player for us for many years, I’m sure.
“He’s a work in progress and we’re working on him day-in, day-out to improve him.”
Barnsley: Steele, Trippier, Shackell, McEveley (Devaney 46), Hill, O’Brien, Doyle, Colace (Butterfield 65), Arismendi, Gray (Bennett 80), Harewood. Unused substitutes: Preece, Lovre, Fearnehough, O’Connor. Booked: Arismendi, Colace.
Reading: McCarthy, Gunnarsson, Ingimarsson, Khizanishvili, Harte, Tabb, Howard (Antonio 78), Karacan, Long, Church (Manset 66), Kebe. Unused substitutes: Andersen, Taylor, Pearce, Morrison, Obita.
Referee : Steve Tanner (Somerset).
Man of the match: Luke Steele.