With Bayern held 1-1 by Nurnberg, the Royal Blues had the opportunity to increase the pressure on the Bavarians in the title race.
But they did not show their true colours, according to Stevens, who has promised a thorough enquiry into his team's shortcomings.
"Explanations are required, but I would rather keep that internal for the time being," the Dutch coach told Sky television.
"I could have taken nine men off, and I did take some of them off.
"And then the only one who actually performed well got a second yellow card (Kyriakos Papadopoulos), which is a shame.
"We were begging to lose this game."
Schalke director of sport Horst Heldt pointed the finger at the players for such an uncharacteristic performance.
"It was a total failure," he said in the Bild newspaper. "We absolutely deserved to lose.
"I cannot remember when we last played so badly."
Andre Schurrle put Leverkusen on the way to victory with a delightful opening goal and he felt the win could have been by an even greater margin.
"I just struck it perfectly and that gave us a boost for the second half," he said. "It was the bottle opener to the game.
"We played really well. In the second half, we could have scored one or two more goals so, therefore, it was a fully deserved win."