It wasn’t exactly a vintage Manchester United performance but it a 1-0 victory was a result they badly needed in the battle of the crisis clubs.
Juan Mata’s 54th minute strike was enough to give United a win in a game that was high on tension but short of any real quality.
It spares Mourinho more pressure with the storms clouds gathering after he revealed ahead of the game that he was struggling to adjust to life in Manchester.
But there will be more questions for Guardiola who has now on his worse streak without winning a game while his last four away trips have also failed to yield a victory.
Two of Mourinho’s previous three victories over Guardiola had come in cup competitions and he will now fancy his chances in the EFL Cup after securing a place in the quarter-finals against West ham.
The Portuguese named a strong lineup in an attempt to win his first possible trophy with his new club — just as he did during his first spell at Chelsea.
But his opposite number Guardiola took a different view, choosing to rest some of his key players such as Sergio Aguero and David Silva with a clash with Barcelona looming on the horizon in the Champions League.
In came Spanish youngsters Aleix Garcia and Pablo Maffeo — both making just their second starts for the club — while backup keeper Willy Caballero replaced Claudio Bravo, who made a nervy debut at Old Trafford in City’s 2-1 win in August.
But it was the same old tactics from the City boss as they try to control possession from the outset. Vincent Kompany, Nicolas Otamendi and Garcia passing the ball comfortably across the back under little pressure.
But despite having more of the ball they lacked a lot of creativity without Silva and the injured Kevin de Bruyne.
They did muster the first opportunity within 90 seconds when Jesus Navas burst down the right wing and fired in a cross that Kelechi Iheanacho headed just over the bar.
And they were unfortunate not to have a penalty when Garcia caught Michael Carrick napping and the England player caught his ankle as looked to get on the end of Nolito’s throughball.
But they were a rare sights of goal for a side lacking some confidence after five games without a win and suffering chastening defeats to Spurs and Barcelona in their last two games on the road.
Not that United look any more positive. It took until the 23rd minute for them to have a shot on target when they were awarded a free-kick 25 yards out for a foul on Carrick. Zlatan Ibrahimovic’s drove his strike into the wall and it dribbled wide to sum a lacklustre first half from the Swede.
He gave the ball away on a couple of occasions as he struggled to impose himself on an inexperienced City side and the home fans were growing impatient with his performance.
But he wasn’t the only one — Paul Pogba was sloppy with his passing in the centre of midfield as Garcia and Fernando busied themselves around him while Marcus Rashford was well marshalled by Maffeo.
The one time Rashford did breakthrough just before half-time, the young fullback made a timely challenge as he threatened Willy Caballero’s goal.
Pogba finally found some space at the second half as United overloaded the attack. Ibrahimovic played in hm but his left shot was tipped onto the post by Caballero — the hero at Wembley seven months ago when City won the trophy on penalties.
But he could nothing five minutes later as Mata swept in Ibrahimovic’s low cross. The Swede finally used his physical advantage to beat Otamendi in the air and when he tapped the ball across, Ander Herrera brushed Fernando off the ball for Mata to score.
Ten minutes later Ibrahimovic missed the cance to double United’s advantage when he failed to make contact with Rashford’s drilled cross with the goal seemingly at his mercy.
Guardiola threw on Raheem Sterling and Segio Aguero shortly after as he tried to rescue the game.
Mourinho’s response was to bring on defensive midfielder Morgan Schneiderlin while Rashford also went off, although he looked like had taken a knock. City pushed the United defence back but never looked like ending a miserable run of never winning a cup tie against United at Old Trafford.
De Gea 6, Valencia 6, Blind 6, Rojo 6, Shaw 6, Herrera 7, Pogba 6, Carrick 6, Mata 8 (Schneiderlin 72, 5), Ibrahimovic 6, Rashford 6 (Lingard 81).
Caballero 6, Maffeo 7, Kompany 6 (Kolarov 45, 6), Otamendi 6, Clichy 6, Garcia 7, Fernando 7, Nolito 6 (Aguero 70, 5), Navas 7, Iheanacho 6, Sane 6 (Sterling 63).