The Parkhead side saw an early lead against Sparta disappear before the half after Odsonne Edouard had given the visitors the lead in Prague.
The hosts netted twice to turn the game on its head and further up the pressure on the Celtic boss, who had supporters calling for his resignation earlier this week.
Stewart said on BBC Radio Scotland: “There’s only so many times you can go to the well in terms of trying to get a reaction from the players, which Neil Lennon has.
“When the reaction doesn’t come, it almost becomes a situation of ‘what else can I do?’ And I genuinely think Neil Lennon will be thinking that and he’s probably thought that for weeks – at a loss to get a reaction for what he needs.
“Celtic have been uncharacteristically very, very soft.”