Spurs travelled to the King Power Stadium on the back of a 1-0 win over champions Manchester City.
But the fifth-placed team, with Conte urging his side on again after gallbladder surgery, endured a miserable afternoon in the Midlands as Nampalys Mendy, James Maddison, Kelechi Iheanacho and Harvey Barnes scored for the struggling Foxes.
Rodrigo Bentancur had given Spurs an early lead but the Uruguayan was later forced off the field with a knee injury just days before Spurs face AC Milan in the Champions League.
"To be consistent is a long process, it's a mental process, you have to be better mentally and better with the approach," Stellini told reporters. "After we scored the goal, something changed.
"We struggled a lot and we are disappointed for that. There's not an explanation because if you know what happened you can change this. It happened last season, after we beat Manchester City we lost in Burnley.
"We are a team, in the team something has to change, not individually. It's about the desire.
"It's about recovering the energy mentally. After a game against Manchester City maybe you used all the energy you had... You have to recreate the same energy."
Stellini was glad to have Conte back alongside him on the touchline but urged his often highly-strung boss to take it easy ahead of some high-profile encounters.
"It's good for the club, the team and everyone to have Antonio back," he said. "He has to take it easy a bit, he cannot use his energy 100 percent and we have to give something more to cover the gap.
"We know we have to be more responsible. We have an important game on Tuesday in Milan."
By contrast, Leicester manager Brendan Rodgers had no complaints about his side's "mentality".
"It was a brilliant performance by the players," he said. "A really brave performance.
"To be as brave as the players were and commit to what they asked us to do it was everything I want in a team -- aggressive, strong, competitive. There were so many great attacking moments.
The former Liverpool boss added: "Now we have the balance in the team, players coming back, we're starting to see (the improvement). We have to keep this mentality and keep this consistency."
