The midfielder began the night of demolition at Celtic Park when his shot was deflected into his own net by the Perth club’s midfielder Liam Craig in the 28th minute and four minutes later Commons grabbed number two with another drive before setting up Gary Hooper for number three before the break.
The Scotland international scored from the spot 10 minutes into the second half to begin an argument of whether or not that was his hat-trick before substitute Charlie Mulgrew, on at the interval for Hooper, completed a night of misery for the hapless McDiarmid Park men when he made it 5-0.
Neil Lennon made five changes for the game with Mikael Lustig, Scott Brown, Victor Wanyama, Tony Watt and Hooper coming into the team which lost 2-0 against Killmarnock on Saturday, with Miku, James Forrest, Adam Matthews, Beram Kayal and Mulgrew dropping out of the side.
Saints showed two changes with David McCracken and Kevin Moon in for Frazer Wright, out with a broken nose sustained in the 1-1 draw with Inverness at the weekend, and Nigel Hasselbaink, who was named among the substitutes.
After a number of near misses, Commons eventually got the breakthrough albeit with the help of Craig whose attempt at blocking the angled-drive succeeded only in taking the ball past Alan Mannus and into the roof of the net.
There was no doubt about Celtic’s second goal which followed soon afterwards.
Lustig’s pass from wide on the right found Hooper who quickly played in Commons and he drilled his shot low past the helpless Mannus from 12 yards.
Steve Lomas’s men looked sapped of all belief as Celtic piled forward at will. In the 38th minute Lustig had a header cleared off the line by Steven Anderson after McCracken had inadvertently headed off his own crossbar.
However, a minute later the tie was effectively over when Hooper, in typical poacher’s fashion, was on hand to knock Commons’ pass past Mannus, who was left exposed once again.
Celtic extended their lead in the 56th minute when the hosts were awarded a penalty after Anderson fouled Watt and Commons, almost inevitably, stepped up to slam his spot-kick past Mannus.
It had become a matter of how many Celtic would score and just after the hour mark Mulgrew made it 5-0 after more indecision in the ragged McDiarmid Park defence had allowed the Hoops utility player to find a yard before shooting into the far corner.
Celtic kept pushing for more goals but five minutes from time, Hasselbaink, on for Gregory Tade, struck the outside of the post, depriving the visitors of a consolation goal.