Salech had not featured since Cardiff’s win over Charlton Athletic before last month’s international break, injuring a shoulder as the Bluebirds registered their first win since promotion back from League One.
Bluebirds boss Brian Barry-Murphy – watching from the stands as he served a one-match touchline ban – was forced into a shuffle and introducing Salech earlier than expected when it emerged former Blues man Bielik needed to be withdrawn.
But, as the rain fell, Cardiff did not look flustered by the change.
Their game of pressure and possession continued as it has done in the recent win over Watford and eye-catching draw at Middlesbrough.
And when Tomoki Iwta refused to let go of Perry Ng at a Cardiff corner, the chance to open from the spot came – only for James Beadle to save Salech’s poorly executed effort.
Salech, though, showed textbook centre-forward play when, reacting to a low cross from Ollie Tanner, he expertly beat Phil Neumann before rolling home from close range.
Salech may have added to his tally, going close with a glancing header from a corner and shotting just wide from a Cardiff high-press.
Cian Asford, too, was denied finishing off a superb sweeping move with Beadle alert to the danger.
Trott, in comparison, had a quiet afternoon until Birmingham finally found their energy and drive in the closing stages.
But he stood tall to keep out Gray from inside the area, with Cardiff holding the late Birmingham rally at bay for a third win of the campaign.