Despite increasing discontent among the fanbase following the weekend's abject loss, a vocal home support still provided their full backing in Govan.
They were stunned within minutes of the first whistle, however, when David Miculescu dispossessed a dithering Jack Butland and rolled in what he thought was an opener, but the goalkeeper's blushes were spared as a foul was given by the referee.
It was a moment that could have spooked Clement's men, but they got the goal their high-octane start deserved when questionable goalkeeping allowed Lawrence to squeeze in a curled effort from the edge of the box.
And it was 2-0 just after the half-hour mark when Cerny won possession in the final third before bending a fine strike into the far corner with his weaker right foot.
On-loan Wolfsburg winger Cerny added his second and Rangers' third after the break when a sweeping counter-attack ended with the Czech forward dipping the shoulder and rolling in a composed finish.
Then there was time for substitute Igamane to open his Rangers account when he dragged one into the far corner after a couple of stepovers.
FCSB, the fifth-placed side in Romania's top flight, tested Butland through Siyabonga Ngezana in the first half with the score at 1-0, but they were simply overwhelmed for the majority of the contest.
A potential knee injury for midfielder Lawrence, who was substituted at the break, plus second-half issues for Cyriel Dessers and Leon Balogun, were the only real negatives for Clement, whose side sit 11th in the 36-team table.