Rangers 4 - 0 St Johnstone
Nikica Jelavic's double helped ensure that Rangers narrowed the gap at the top of the league to five points.
Jelavic fired the opener following Rangers' first spell of pressure after four minutes at Ibrox.
Jody Morris forced Rangers goalkeeper Allan McGregor into a superb save before Lafferty headed the home side's second four minutes before the break.
Papac's fine finish made it 3-0 on 81 minutes and Jelavic swept in another for the home side in stoppage time.
And the victory ensured that the Ibrox men capitalised on Celtic's slip up earlier in the day.
Combined with the Scottish Premier League leaders' 2-0 defeat away to Motherwell, the reigning champions are now five points behind with two games in hand.
And it completed a heartening few days for Walter Smith's side, who responded to last Sunday's defeat at Celtic Park with progress in the Europa League.
Rangers captain David Weir had been rested after overcoming illness to play against Sporting on Thursday, the veteran's place at the heart of the defence being taken by Kyle Bartley.
The introduction of Kyle Lafferty, Vladimir Weiss, Nikica Jelavic and David Healy added to the home side's attacking options, with El-Hadji Diouf, Richard Foster and John Fleck being the others to drop out.
Saints made one change following their draw with Dundee United, Stevie May replacing the injured Jamie Adams in the starting line-up but with new signing Danny Invincibile only finding a place on the bench.
The day had already started well for Rangers, with Celtic's defeat by Motherwell being completed at Fir Park less than half an hour before kick-off.
And it only took another four minutes for the Ibrox men to add an opening goal to the celebrations.
A shot from Maurice Edu - who netted the decisive goal in the Europa League - was blocked and Jelavic was the first to react and pounce on the loose ball before dispatching an unstoppable volley past the grasp of Peter Enckelman.
There were suggestions that the ball had struck the hand of Rangers defender Madjid Bougherra in the build-up, but referee Steven McLean pointed to the centre circle.
Lafferty attempted to add to the scoreline, but the St Johnstone goalkeeper was up to the task this time and able to smother at the near post.
Saints lost the influential Michael Duberry to injury after 17 minutes, with Alan Maybury coming on and slotting into the reshuffled defence.
Rangers were threatening again when Lafferty whipped a cross into the box from the left that was missed by Jelavic and fell for Healy, but the Northern Irishman's shot was blocked well by Enckelman.
The home side had to make a change of their own when Weiss limped off and youngster Gregg Wylde was thrown into the action after 27 minutes.
Lafferty then squandered a great chance when he collected a lovely pass from Steven Davis only to hook well over when he should at least have tested the goalkeeper.
The big striker fired straight into Enckelman's arms when Lafferty remained onside with only the goalkeeper to beat, with Davis again the provider.
St Johnstone had rarely threatened but would have equalised if Rangers goalkeeper Allan McGregor had not produced a breathtaking save from a fine Jody Morris drive.
Cleveland Taylor then sliced just wide from 18 yards as the Perth side and went close.
Lafferty eventually succeeded four minutes before the break when he rose above everyone else in the box to meet Wylde's corner and sent a firm header into the back of the net.
It was back to the inaccurate Lafferty after the break, curling an effort high and wide after a cut-back from Jelavic.
Rangers were looking comfortable, but Murray Davidson came close to forcing them into a re-think with an effort into the side-netting.
Saints were beginning to dominate long phases of play, but Edu sent a powerful low drive just wide as the home side responded on the break.
Liam Craig went close for St Johnstone with a header, but Papac ensured there would be no fightback by finishing superbly after playing a one-two with Jelavic before scoring his fourth goal in six games against the Saints.
There was still time for Rangers' to have penalty claims rejected when Enckelman came off his line to clatter Wylde.
However, the winger did have a hand in a goal, cutting the ball back for Jelavic to score in the dying seconds.