The sub-plot to this tie may have been the sibling rivalry between Paul Pogba and his elder brother Florentin, but Zlatan Ibrahimovic stole the show with a hat-trick to put Manchester Unite don course for the last 16 of the Europa League at Old Trafford.
The first wasn’t the prettiest, a deflected freekick from a man who had a habit of scoring against St Etienne during his time in France.
The second arrived 15 minutes from time as Ibrahimovic slid the ball into an empty net when goalkeeper Stephane Ruffier could only palm a cross from substitute Marcus Rashford straight to his feet.
The Swede struck his free-kick low and managed to catch the St Etienne wall out as it deflected and trickled into the net
And any St Etienne hopes were effectively killed off in the 88th minute when the Swede fired his third from the spot after he was bundled over by Kevin Theophile-Catherine.
It gives United a commanding advantage to take to the Stade Geoffroy-Guichard next Wednesday against St-Etienne side who could otherwise have been very dangerous.
As much as Mrs Pogba would prefer it to finish with honours even between her two boys, there must be one winner. Yeo sat watching from a private box last night dressed in a half and half shirt to emphasise her shared loyalty, but it looks like Paul will be the one going through to the last 16.
It had been 40 years since these two clubs had met in Europe, strangely at Plymouth Albion’s Home Park ground due to crowd disturbances involving English fans in the first leg in France.
United prevailed that night and began here as favourites to do so again. Mourinho’s side had lost just once in 22 games in all competitions and held a 100 per cent record at Old Trafford in the Europa League.
St-Etienne, on the other hand, had not been beaten in 10 games in Europe this season and arguably posed a greater threat than any of United’s group opponents, Feyenoord, Fenerbahce or Zorya Luhansk.
Having confirmed his back-four selection in Tuesday’s pre-match press conference, Mourinho only sprang a couple of minor surprises with the rest of his line-up.
Goalkeeper Sergio Romero was given his fourth outing in Europe this season and Marouane Fellaini came into the side for Henrikh Mkhitaryan who was ill.
There was no rest, however, for Ibrahimovic who had scored 14 goals in 13 games against St-Etienne during his time at Paris Saint-Germain and was not surprisingly singled out by the French fans.
Paul Pogba covers his mouth as he speaks with his brother Florentin, a defender, as the siblings did battle at Old Trafford
The Pogba brothers duly squared up wearing the No 6 and 19 that Florentin had shaved into his hair beforehand, and it was to them that the cameras inevitably turned as the two men embraced in the tunnel beforehand and proceeded to walk out on to the together chatting happily.
They were greeted by green and red flares from the away fans that temporarily left Old Trafford cloaked in smoke, and it was United who started as if they were playing in a fog.
Twice in the opening three minutes, Eric Bailly had to be bailed out by his teammates after needlessly giving the ball away.
Chris Smalling got across to intercept Kevin Monnet-Paquet’s pass and then Romero left his goal unguarded to close down Romain Hamouma who skipped past the United keeper and tried to find a teammate but was thwarted by Daley Blind.
It was symptomatic of a sloppy start by the home side but they finally out a good move together which allowed Anthony Martial to bring a smart save out of Ruffier from 25 yards.
And four minutes later United were ahead. It owed much to the knowhow of Ibrahimovic who convinced Czech referee Pavel Kralovec that he had been fouled by Jordan Veretout, throwing himself theatrically to the ground when in fact there had been little contact between the two players.
The Swede took the freekick himself and opted to drive the ball low through the wall from 20 yards. It rattled between the ankles of Vincent Pajot like a pinball and send Ruffier the wrong way, trickling agonisingly over the line as if in slow motion as Loic Perrin gave pursuit and Ibrahimovic stood with his arms aloft.
Pogba, the one in red, was beginning to exert a growing influence on the game and it was his weighted pass into Juan Mata that so nearly led to a second, the Spaniard swivelling to unleash a volley that was beaten away by a fine save.
But St Etienne still posed a real threat and they had already had a penalty appeal turned down for Blind’s challenge on Jorge Intima when Henri Saivet lifted a shot inches wide after a fine run by the excellent Hamouma.