Commons came off the bench to secure the victory against St Mirren after the Hoops had largely served up a drab affair at Parkhead.
Title rivals Rangers will be seeking to reduce the deficit again when they travel to Hamilton on Sunday.
Celtic made three changes from their midweek win over Hibernian as Cha Du-ri, Freddie Ljungberg and Ki Sung-yueng all started at Parkhead, with Mark Wilson and Joe Ledley named on the bench and skipper Scott Brown absent from the squad.
Brown, Wilson and Georgios Samaras were all one booking away from suspensions which would rule them out of the final Old Firm game of the season, perhaps explaining their omission against Saints.
Despite an impressive midweek win over Aberdeen, Buddies boss Danny Lennon rang the changes by adding Kenny McLean, Lee Mair, Jim Goodwin, Marc McAusland and Paddy Cregg to the side. Paul McGowan was ineligible under the terms of his loan agreement and Craig Dargo was injured, while David van Zanten, Aaron Mooy and Jure Travner were named among the substitutes.
St Mirren had the chance to apply some early pressure when they won a free-kick just outside the box but Michael Higdon's effort failed to make it past the Celtic wall, before the ball broke for McLean who blasted wide.
At the other end, Gary Hooper pounced on a James Forrest cross only to see his close-range shot blocked by goalkeeper Paul Gallacher but the offside flag was raised in any case.
Real chances had been few and far between but Celtic were threatening when Cha provided a dangerous cross from the right flank that was met by Anthony Stokes, who saw his header flash inches wide of the target.
Hooper netted a double in midweek but had failed to make a similar impact against St Mirren and he was replaced at the interval by Shaun Maloney, who made his long-awaited comeback after four months on the sidelines with an ankle injury.
Celtic were out of the traps quickly after the restart and Stokes could have opened the scoring with a drive from the tightest of angles that Gallacher did well to block at the near post, before Maloney fired a tame shot wide.
The home side continued to struggle in their attempts to break down their stubborn visitors and made a second change with an hour gone when Ki was withdrawn for Ledley.
Commons was thrown into the action in place of Ljungberg with 20 minutes to go and his introduction had the desired effect for Celtic.
Maloney was the provider with the pass that was collected by Commons 12 yards out before he lashed past the grasp of Gallacher with 79 minutes on the clock.
Saints could have snatched a late leveller but John Potter fired wide in the final minute with their best chance of the game.