"The players can't believe how we lost that match 4-1, but I think the referee decided the outcome," Levein said after the game at Bramall Lane.
"The ball spun up and hit Nils-Eric Johansson on the shoulder. It's such a critical decision because it looked like we were going to win.
"It's come at such a crucial stage of the match, and it's cost us today because it looked like we were going to win the game and that was the turning point."