The substitute squandered a golden opportunity to earn all three points, but a 1-1 draw means they still have a slim hope of qualifying for the round of 16.
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Celtic's Georgios Giakoumakis gave the home side the lead on 34 minutes, before the Scottish champions were pegged back.
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Arsenal target Mykhaylo Mudryk restored parity for the away side, with a stunning solo goal.
The 21-year-old picked up the ball from 40 yards out, cut inside and then unleashed a fierce effort from the edge of the box, leaving Joe Hart with no chance.
However, the moment to take the lead came, and it surely, was a moment to forget which came on 72 minutes.
Sikan, who came on as a substitute 15 minutes before, had the goal at his mercy from just a few yards.
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The forward, with plenty of time and space, should have taken a touch and rolled the ball into the empty net.
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But, inexplicably, his first touch got away, and the ball eventually bundled out of play for a goal kick - to the delight of the Celtic fans who cheered, and provided plenty of stick towards Sikan after his howler.
"It is the worst miss I have ever seen in the Champions League," said talkSPORT reporter David Tanner.
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"I don't know whether he was caught in two minds, but he miscontrolled the ball and instead of putting it into the empty net the ball dribbled past.
"Celtic were let off the hook, but what a miss. You will see that on clip shows until the end of time."
Sikan's miss means his side need a win in their final game to go through, while their opponents Leipzig need at least a point to progress to the last 16.