The Ladbrokes Premiership match was held up for a number of minutes while the Czech goalie was given the once-over by the physio, with the Hoops going on to win 2-0.
The former Slovan Liberec No.1 was visibly alarmed by the loud bang from the pyrotechnic device and Kearney - who had used all three substitutes - admitted he feared Hladky would be unable to continue. Goalscorer Ryan Christie was also affected by the incident.
Kearney said after the match: “I had a brief chat with [Hladky] but we’re just thankful that it wasn’t more sinister. I was standing 50-60 yards away and it made me jump so I dread to think how loud it would have been for him.
“I could only see what you could see so I thought he might have to come off, which isn’t ideal.
“We’ll check him out in the morning. We’d used all our substitutes by then and it’s crazy to think we might have had to use an outfield player in goal as a result of that. “You’re tempted to walk your team off the pitch...”
His opposite number Neil Lennon was similarly dismayed by the incident - the latest in a long line of disorder at Scottish football matches.
The interim Celtic boss added: “It serves no purpose in a football ground. I don’t see the motivation for anyone to bring a flashbang or pyrotechnic into a stadium. The health and safety of the players is paramount.
“It didn’t just spook the St Mirren ‘keeper, but some of our players too. The goalie could have burst an ear drum. It delays the game when we have scored a goal and want to get going again.
“I don’t understand why people want to do that. There is no logic in it. Why does an individual want to bring that to a game? It doesn’t impress anyone, it doesn’t add anything.
“It just damages people - and it could have damaged a player.”
Christie, interviewed after the game, echoed his manager, saying: “Tonight was a bit disappointing. I feel for [Vaclav], it could have caused a serious injury.”
Discussing the incident after the game on BT Sport, former Celtic striker Chris Sutton called for action as he said: “I don’t see what the person or persons who did that think it adds to the game.
“We need to stamp it out; we need hardline action.”