Stuttgart progressed 4-2 on aggregate, but the night in Glasgow left Undav furious after two second-half goals were ruled out for offside following Video Assistant Referee intervention.
The only goal of the match came after just 28 seconds, when Celtic midfielder Luke McCowan struck early to give the hosts hope. Stuttgart, however, held firm over the two legs to secure qualification.
"The most important thing is that we progressed," Undav said after the match. "I'm angry, but I'll keep that to myself."
The Germany international made it clear his frustration centred on VAR. Stuttgart thought they had found the net twice after the break, only for both efforts to be chalked off following reviews.
"VAR is supposed to be there to help the game," Undav said. "But now it makes almost more mistakes than the referee on the pitch."
He went further in his assessment of the officials overseeing the technology. "Sometimes the people in Cologne - the VAR - seem to be watching with their eyes closed. It's getting more and more ridiculous what nonsense VAR produces. I don't even have an opinion on it anymore. It's just nonsense."
Despite the controversy, Stuttgart achieved their primary objective. Over the two legs they proved clinical enough to build a decisive advantage, and disciplined enough to withstand Celtic's early surge in the return fixture.
In knockout football, progress is everything. Undav's anger was clear, but Stuttgart marched on - and that, in the end, was what mattered most.
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