Hasenhuttl’s side had been in possession when play was stopped early in the second half due to an injury to Jamie Vardy but referee Jon Moss then presented the ball back to the hosts, leading to Maddison capitalising.
“I still must say that I cannot follow what happened there at that time,” said the Saints manager, previewing Saturday’s clash.
“This was definitely decisive for this game. It’s always a motivation when you lose a game but when you lose it in this way even more.
“The way was absolutely not right and not fair. We got punished for something that was not right.
“It’s gone and we cannot win this game. But we can win this game on the weekend and this is the goal and this is why we’re going there with all of the motivation we need to have.”
Saints forward Che Adams will be fit to face the Foxes after suffering a minor injury in last weekend’s 2-2 draw at home to Leeds.
Romain Perraud, Tino Livramento and Theo Walcott remain unavailable.