The Scottish champions were easily defeated by their Czech visitors, who haven’t played a league game in over a month due to lockdown measures in the Czech Republic.
Lukas Julis grabbed a hat-trick while Ladislav Krejci added a fourth in injury time. Celtic did get back into the game at 2-1 thanks to a strike from substitute Leigh Griffiths, but were way off the pace and now sit bottom of Group H in the Europa League.
Lennon told BT Sport: "I don’t know where that performance came from and I can only apologise to the supporters. The players know that is totally unacceptable. The hungrier team won tonight and that’s on the players.
"I’ll have to change the culture. We’ll need to have a sea change over the next couple of days. We go from playing very well in the semi-final to such an insipid performance. I defend the players to the hilt, but I can’t defend that.
"Fingers will point at the manager and that’s fine, but I set out a strong team tonight. Sparta thoroughly deserved it and that’s hard to say.
I can’t defend that tonight.”
The Celtic manager was asked for his reaction to Mohamed Elyonoussi, subbed off in the second half, caught on camera appearing to check his phone while the game was still going on.
He responded: “He better not have been. That’s something I’ll deal with.”