Clinical Celtic warmed up for the Nou Camp with what turned out to be little more than a training exercise as they outclassed a feeble St Mirren.
The Buddies contributed to their own downfall as the unmarked Gary Hooper bundled in the 15th-minute opener.
Goalkeeper Craig Samson's fumble allowed Efe Ambrose to volley in the second three minutes later.
Victor Wanyama's half-volley and a 25-yarder ended the contest by the break before Tony Watt's late header.
Celtic head for their Champions League Group G visit to Barcelona on the back of a sixth straight victory that stretches their lead in the Scottish Premier League to five points ahead of the afternoon's later fixtures.
They do so having justified manager Neil Lennon's decision to make five changes as he rested some of his squad nursing injuries and international duty fatigue.
Opposite number Danny Lennon had replaced striker Sam Parkin with defender David Barron, admitting before kick-off that his experiment of using three strikers did not work last time out against St Johnstone.
The Paisley side started confidently as they sought their fifth straight home victory, but their defence could not cope with Celtic's own three-pronged attack.
However, the Buddies could have been ahead when David van Zanten's cross pinpointed Lewis Guy and the striker's controlled volley from 12 yards forced Fraser Forster into a save at the second attempt.
Any home optimism that engendered was quickly quashed as Lassad's low cross found the almost apologetic Hooper, who looked quizzically for an offside that never came after the ball travelled the four yards towards goal off his shin.
Ambrose provided a far more spectacular celebration of his first Celtic goal with a quadruple somersault following his close-range volley into the roof of the net after Samson let a Charlie Mulgrew free-kick slip out of his grasp.
Kenny McLean was the one player impressing for the Buddies, but the Scotland Under-21 midfielder should have burst the net from 15 yards instead of firing straight at Forster after being set up by Guy.
Wanyama effectively killed off thoughts of a St Mirren fightback after 32 minutes, the midfielder volleying low through a sea of legs from just inside the penalty box from a badly-defended corner.
The Kenya midfielder bettered that six minutes later with a beautifully controlled side-foot drive from all of 25 yards into the far corner.
With Mulgrew and Ambrose having also missed gilt-edged chances, angry St Mirren boss Lennon hauled off the hesitant Barron and replaced him with winger Dougie Imrie at half time.
Kelvin Wilson came off for Celtic as a precautionary measure, with Emilio Izaguirre his replacement, after the defender picked up an injury that is not thought to threaten his participation on Tuesday.
Celtic continued to create chances, with Beram Kayal forcing a near-post block from Samson, who then turned a Joe Ledley drive wide.
St Mirren were more in possession without the SPL's joint-second-top goalscorers managing the consolation of their first goal against Celtic in six games since the 4-0 win that led to the departure of Tony Mowbray as manager in March 2010.
Steven Thompson forced a goalline block by Forster, headed wide and volleyed over, but Celtic were to have the last word when Watt headed in Izaguirre's header with six minutes remaining before Wanyama was denied a hat-trick by the crossbar.