Just when an exquisitely-taken first-half goal from the Scotland Under-21 skipper looked set to hand the Tannadice side their first victory here since Boxing Day 1992 - when a certain Duncan Ferguson grabbed the winner - a combination of some errant finishing by Brian Graham and some eccentric goalkeeping from Radoslaw Cierzniak permitted Charlie Mulgrew to head in the equaliser which avoided any unnecessary pre-Amsterdam embarrassment for the hosts.
A draw might have been just about fair on the balance of play, but the rights and wrongs of the situation would have meant little had Graham not got himself in a muddle when he hared onto the end of Nadir Ciftci's expert cross with a chance to double the visitors' lead in the second half, or had United's Polish goalkeeper found a team-mate rather than wastefully cleared straight to Emilio Izaguirre on halfway in the second minute of injury-time.
Indeed, after 21 years without one, United's hopes of holding on for a famous victory at Parkhead were effectively gone in six seconds. Tannadice skipper Gavin Gunning revealed that his Polish goalkeeper had hurried his clearance when he saw referee Alan Muir counting the time he was holding onto the ball to his alotted six seconds. Play duly swept to the other end, and before long Mulgrew was directing Emilio Izaguirre's cross into the bottom corner.
"I didn't think it was a very good kick out," said Gunning. "A minute to go and he's panicked because the referee is counting on his fingers to six seconds.
"He's never going to give a free-kick there, the referee, so if Rado had just taken a few extra seconds I think we would have had three points. But he's been quality this year and I don't want to give him any flak.
"As for Graham, I don't think he needs to be embarrassed because he played really well out there," the Irishman added. "Although he will get bit of stick on Monday morning. I think the cross swerved a bit, or that's what he's saying anyway."
For Jackie McNamara, back on his old stomping ground in the east end of Glasgow, they were all signs of the youthful naivete of his talented side -as was the sight of left-back Andrew Robertson charging up the right wing late on in search of a second goal.
"In injury-time our left-back has nearly got down the right-hand side at 19-years-old," he said. "It is enthusiasm, but sometimes you just see it through and be professional."
It had taken a late free kick from Anthony Stokes to separate these sides when they met earlier in the season at Tannadice and the hardly souls who braved the biblical rain in the East End of Glasgow were rewarded with another evenly fought contest.
With one eye on Amsterdam, Neil Lennon rewarded Amido Balde with a rare start, and gave young Darnell Fisher another chance at right-back.
United used the appeals system to their benefit by fielding Nadir Ciftci while he awaits final sanction for alleged violent conduct and they must have been glad they did: this was an accomplished performance from the young Turk.
From the start, both sides proved capable of creating chances. Gary Mackay-Steven forced Fraser Forster into some early work at one end, then John Souttar headed a goalbound Stokes drive away at the other.
When it came, the opening goal was as good as any you will see all year; United keeping possession well before Ciftci's sweetly scooped pass was perfectly timed for Armstrong to control it on the run, round Forster and squeeze it in at the post.
Celtic, who introduced Mikael Lustig, Teemu Pukki and Derk Boerrigter as the match wore on, continued to create occasional openings too. Stokes might have had a couple before the first half was out, even if all hope seemed to have gone when Cierzniak saved well from the Irishman with just minutes left.
But thanks to a mixture of their own belief and the individual frailties of their opponents, that unbeaten league record lives to fight another day