Initially referee Nigel Hegley took no action but after his assistant's signal he awarded a free-kick on the edge of the area and showed Kenny a yellow card when most spectators expected to see red.
"The decision not to send off their goalkeeper Paddy Kenny off changed the whole tone of the match," Coppell said after the Royals 2-1 win.
"Virtually everyone in the stadium - Reading fans, neutrals and Sheffield United fans - would have thought it was an automatic red card for handling outside his area, and the referee got it wrong.
"The match was distorted from then on, because it would have been a totally different game - especially as United didn't have a reserve keeper on the bench.
"Last year people said that if teams stopped Dave Kitson, they stopped us.
"But I hope we have a bit more firepower from different areas now. Brynjar Gunnarsson ghosted in very nicely for both goals - and it was a really good feeling when the ball went in."