Jelavic and Lafferty were both restored to Walter Smith’s starting line-up as two of four changes to the side that started in Lisbon three days earlier.
Also back in the fold were Vladimir Weiss and David Healy, with David Weir, Richard Foster, El Hadji Diouf and John Fleck dropping to the bench.
There was a sense of anticipation as kick off approached with supporters and players alike aware that their Old Firm rivals had dropped points at Fir Park in the lunchtime kick off.
And Rangers made sure they had a strong start to take advantage of that with Jelavic slamming home a fine finish with his left foot.
Mo Edu’s shot was initially blocked but the Croatian forward reacted fasted and thundered the ball past Peter Enckelman.
It was the highlight of the early stages as St Johnstone offered little in attack and Rangers probed to find another opening against a packed visiting defence.
The rhythm of things was broken a little by two forced substitutions in the opening half, Michael Duberry apparently suffering a groin injury and Vladimir Weiss taking a kick on the top of his foot.
Alan Maybury and Gregg Wylde respectively replaced the pair and Wylde would play a crucial part in the Light Blues’ second.
It was his corner which had the pace and curl to elude the Saints defence and find Lafferty who nodded home with five minutes of the half remaining.
It was a welcome boost for the Ulsterman who had come in for a bit of criticism from the home support after a few squandered opportunities.
On more than one occasion he found space on the left only to waste the chances by opting to shoot when he might have passed.
The one time he did opt to pick out a team mate it was international colleague Healy, who was expertly denied by Enckelman.
Lafferty may have been looking for Jelavic but when the Croatian couldn’t connect it fell to Healy, whose effort was true, but Enckelman got down well to save with a foot.
Not to be outdone by the visiting keeper Allan McGregor produced the best save of the half when he tipped round Jody Morris’s well struck effort a few minutes before the break.
McGregor was rarely troubled in the second period as Rangers closed out the fixture without reaching the heights of their last home SPL match against Motherwell.
Lafferty was again guilty of wasting an opening in the early stages as the Light Blues continued to probe for more goals to relieve any lingering nerves about the result.
With the Gers struggling to get behind a deep lying defence, despite the pace of Wylde, Edu tried his luck from distance on the hour mark.
There was plenty behind the rasping left foot effort but it drifted a little wide of Enckelman’s upright.
It seemed that things would drift to a conclusion with John Fleck’s introduction seeing another change in shape for the home side.
But just as the home fans eyed the exits Papac popped up with his fourth goal against the Saints.
He latched on to Jelavic’s excellent back heel and with time and space in the box he slotted comfortably past Enckelman.
The Gers should have had an opportunity for a fourth moment later when Fleck skipped beyond the Saints keeper only to be brought tumbling to the turf.
Referee Steve McLean ignored the home side’s claims for a spot kick however to the bemusement of Walter Smith in the technical area.
They almost carved another in the final minutes however as Davis slotted in Whittaker on the right and the full back’s cutback found Jelavic.
Enckelman was equal to the Croatian’s effort on that occasion but he could do nothing to stop the striker bagging his double in injury time.
When the ball broke to him off the Saints keeper he slammed it in to the corner to cap a perfect afternoon for his side.