It was an opening 45 minutes that was largely dominated by Ronny Deila's Celtic, who are bidding to win the trophy for a 15th time.
There was just four minutes on the clock when Celtic centre-back Virgil van Dijk rose highest to meet a corner with a header which beat United goalkeeper Radoslaw Cierzniak, but Sean Dillon was on hand to hack the ball clear off the line.
Next to try his luck for the Bhoys was Leigh Griffiths, but, having had his shooting angle narrowed by Cierzniak, the striker could only find the side-netting.
It took United to go down to 10 men for Celtic to break the deadlock. With Dillon receiving treatment for an injury, Commons broke free and, after seeing Cierzniak turn his initial effort on to the post, the midfielder tucked in the rebound from close range.