SPARE a thought for referee Kevin Clancy.

When he prepares to signal the start to Celtic's league game against Dundee United on Saturday, he'd be better keeping his whistle in his pocket and just hollering: Seconds out, round four.

After last night's replay mayhem - which saw Anthony Stokes and Ryan McGowan sent off in separate flashpoints in the dying minutes as the Hoops won 4-0 - this series of games is becoming more of a war of attrition than a festival of football.

The previous two meetings saw four men shown red, three penalties, two appeals and one Notice of Complaint.

It set things up on a knife edge for last night's Scottish Cup quarter final replay.

And it had appeared to be passing without any real trouble, but, just when you thought it was safe to come out from under the blanket ...

Boom! Red for Stokes for retaliation on Paul Paton, with the Celtic striker claiming he had been elbowed, and pointing to blood dripping from a mouth wound.

Boom! Red for Ryan McGowan as he left substitute Liam Henderson in a heap on the ground on the United goal-line with a scything tackle.

Boom! A melee involving most of the players left on the pitch which ended with John Guidetti and Emilio Izaguirre cautioned.

The flash, bang wallop finish was completed with Virgil van Dijk knocking home goal No.4 to end the rout and send Celtic into the semi-final against Inverness next month.

But that match will have to go some to eclipse last night's rip snorter. As the notorious Old Firm series of games in 2011, which sparked unwanted and unnecessary intervention from Holyrood, proved, familiarity can breed contempt.

Certainly, the low attendance for a game with so much riding on it, pitched at just £15, and blocked from any TV coverage due to the fact it was a Champions League night, indicated the supporters have had just about their fill of this fixture for now.

As for the combatants? Well, to be honest, from the first of these four-games-in-14 days between Celtic and Dundee United, the ill will has been there for all to see.

Countless grievances and grudges have been given no time to heal between meetings.

At Tannadice, then Hampden, the supporters were never afforded the luxury of watching 11 v 11 for a full game.

Few would have bet on a full complement going the distance this time around.

They could have been cashing in within the opening minute, had Calum Murray - the ref handed the poisoned chalice for this replay - not been in a lenient mode.

Radaslow Cierzniak feared he was going to see red after rushing from his box and upending Leigh Griffiths as he beat him to a Kris Commons ball over the top.

But the ref deemed at least one defender had got back between the striker and goal and elected only to caution the keeper.

Perhaps Murray was still stunned by how the tone had been set from the first toot of his whistle, Scott Brown fouling Nadir Ciftci - Jackie McNamara's idea of the best player in Scotland - before he had even had the chance to leave the centre circle after United kicked off.

The Brown/Ciftci sideshow was worth the admission money alone, sticking to one another like foes reunited.

At one point, the Turk actually appeared to make his plea for clemency before he made his tackle from behind on the Hoops skipper.

Later, Ciftci leapt in anticipation of a Brown tackle which never came, a moment which brought a wide grin to the face of the Celtic warrior.

Unfortunately for United, too many of Ciftci's team-mates were also posted missing in action, although Ryan Dow did carry enough threat in the early stages to entice Efe Ambrose into a foul which saw him pick up a booking.

The Nigerian was a relieved man when the ref did not got for his card again early in the second half after Ambrose tripped Ciftci.

That was during United's best spell, Craig Gordon pushing out an awkward Ciftci shot after McGowan had got to the goal-line and fizzed the ball across the face of the Celtic goal.

It was, in the main, huff without puff, however. United may be sponsored by Calor, but they were definitely not cooking with gas, and Celtic turned up the heat on their visitors early.

The opening goal came after just 17 minutes, Stokes sending a low-trajectory free-kick into the area from a central position which Jason Denayer got to ahead of Jaroslaw Fojut, his header looping over the giant Pole and the despairing leap of Cierzniak.

The keeper did have more success twisting and stretching to turn a James Forrest shot-cum-cross on to the face of his crossbar, preventing the Hoops from doubling their lead and, in all likelihood, finishing the tie before the break.

At the other end, Gordon was fortunate when another low driven cross by McGowan slipped through his grasp, but deflected enough to run behind the in-rushing Ciftci.

It was one of those nights for United, who have now followed up a win against Celtic in December, with a Scottish Cup draw, a cup final defeat and another in this replay.

Confirmation of this latest second prize was delivered by Griffiths 12 minutes into the second half - as Guidetti stood on the sideline waiting to take his place.

Brown broke into the United half on the right wing, looked up and sent a ball over the top to the darting striker.

Showing excellent technique, Griffiths took the ball as it came over his shoulder and volleyed it beyond the rooted Cierzniak.

With 11 minutes remaining, Commons played a one-two with Guidetti, and coolly lifted his shot high into the net to deflate United even further, and Van Dijk rubbed salt into their wounds with the final tap-in goal in stoppage time.

The consolation for the United players is they don't have long to wait for the chance to gain revenge, returning for their league meeting with the Hoops on Saturday.

The chances are, though, they are already sick of the sight of this dear green place.

Friends re-United, right enough.