After a run of three successive wins, Barnsley went behind in the 18th minute when Yakou Meite struck from close range.
Top scorer Cauley Woodrow could have levelled but squandered several first-half chances.
Reading increased their lead on the hour when George Puscas raced through to fire home.
Barnsley lie five points from safety in the Championship and Struber said: “I’m disappointed. The first half was okay, we made good chances but we didn’t score.
“In the first 20 minutes, we had control on the ball and good positioning. But in the whole game, we did not have the right energy for sprints or for the right moves.
“In the end, we did not have the big conviction that we needed so that we can win away against Reading. Some players were only at 90 per cent – or lower – and that was not enough so that you can win this match.
“The last games for us were very hard and I think that we lost energy from them.
“In a normal situation, I think that we can bring more intensity. But we had no intensity, no high-speed running. This was my feeling.
“It was not difficult for our opponent and this was the problem for us. Without this energy, it was just not enough. We had created very good performances in the last three games, we had a high aggression. But today the battery was low. We were not ready for a big fight here. It was too easy for our opponent.”
Reading: Rafael Cabral, Yiadom, Miazga, Morrison, Obita, Meite (McCleary 64), Rinomhota, Swift (Pele 79), Ejaria, Olise (Masika 71), Puscas. Unused substitutes: Walker, Baldock, Blackett, Richards.
Barnsley: Collins, Sollbauer , Halme, Jordan Williams, Ludewig, Ritzmaier (Bahre 57), Mowatt, Oduor (Thomas 46), Woodrow, Brown, Chaplin (Schmidt 57). Unused substitutes: Ben Williams, Dougall, Walton, Simoes.
Referee: J Simpson (Lancashire).