Morelos scored soon after his entrance just before the hour mark, replacing Jermain Defoe who had shone in his absence through suspension.
It was far from a vintage display from Gerrard’s team against a Kilmarnock side who were robust and defensively diligent under interim boss Alex Dyer.
Although Defoe’s performance and goal against Hibs last Friday made a powerful case for him retaining his place in the side, it was still something of a surprise that top scorer Morelos began the afternoon on the subs’ bench.
Gerrard’s selection decision was almost justified after just three minutes when Rangers’ first attack of the afternoon saw the ball worked into Defoe but the veteran striker’s shot in a crowded penalty area was deflected just wide.
With an early breakthrough eluding them, it quickly turned into a bit of a slog for Rangers. For all that they were largely camped in the Kilmarnock half of the pitch, they found clear cut opportunities difficult to carve out against opponents who have reverted to the successful tactical template established under Dyer’s erstwhile gaffer Steve Clarke.
They were content to soak up pressure and wait for chances to launch counter attacks, the first of them causing a scare for the hosts when left-back Niko Hamalainen burst forward to collect Rory McKenzie’s pass and fire in a shot which took a deflection off James Tavernier before sliding narrowly wide.
As Rangers tried to step up the tempo, a fine crossfield pass from Connor Goldson found Ryan Kent in a rare pocket of space on the left. The winger had time to measure his low cross to Joe Aribo whose shot clipped Killie defender Alex Bruce and went wide.
Ryan Jack then flashed a long range effort beyond Laurentiu Branescu’s right hand post, the Romanian ‘keeper not for the first or last time infuriating the home fans with the amount of time he took to restart proceedings from the goal kick.
For all of the frustration directed towards Branescu for time-wasting, the bald fact of the opening 45 minutes was that the hosts had failed to force him into a single save.
Glen Kamara’s long range effort which clipped the top of the crossbar two minutes before the interval was as close as Rangers came in an instantly forgettable first half.
There was a greater sense of urgency to Rangers’ work after the break and Branescu was finally called into meaningful action in the 52nd minute when he got down well to hold a sweetly struck shot from Borna Barisic.
Tavernier then blazed a shot over after some intricate build-up play down the right from the hosts who, despite their continued dominance of possession, had to remain alert to Killie’s occasional counters, such as when Allan McGregor had to make a decent save to keep out Alan Power’s strike from 22 yards.
Branescu did well once again to deny Kent then sprang sharply to his left to save a good attempt from Jack as Rangers’ intensity levels continued to rise.
But it was the introduction of Morelos which provided them with the cutting edge they so badly needed. Just seven minutes after he had replaced Defoe, the irrepressible striker peeled in behind the Kilmarnock defence to beat their offside trap and latch onto a sharp pass from Joe Aribo. There was little doubt about the outcome as he drove the ball beyond Branescu to relieve the tension building up among the home support.
Kilmarnock had no choice but to rediscover a sense of adventure for the closing stages and substitute Chris Burke’s cross set up a good chance for Dario Del Fabro, his header held by McGregor.
Morelos squandered an opportunity to double his tally and ensure a more relaxed finale for Rangers when, having been played in by Tavernier, he attempted to take the ball around Branescu instead of shooting first time. But Morelos had done his job as Gerrard’s side saw out another important victory.